In this guide, I've included every mainstream storyline boss (save maybe one or two) in Kingdom Hearts 2.  Some of the strategies are longer for tougher bosses, and shorter for easier bosses.  Regardless, there is still something for everybody.  Additionally, I'll include the difficulty of each boss based on my experience. The system will be based on this scale:

1-2=Very easy
8-9=very hard

Boss: Twilight Thorn
Difficulty: 2
Strategy: This boss isn’t too tough to beat. The key is being aware of the reaction commands when they appear. At the beginning he will trap Roxas, and there will be a chain of three reaction commands. The first two happen near the beginning of the series, then when the world tips, use the third when prompted. After this, start hitting the head until it recovers. Now during this part of the battle, you must strike him when his head is low and his arms are swinging wildly. When he uses electric waves, use the reaction command to ride them up to his head and hopefully get a few hits in. You will have to be quick about it though, because the waves don’t last for long. He hast two attacks that you cannot counter, but you can avoid them. The first is when he cartwheels in place. Jump over his arm to avoid this one. The second attack is when he back flips and swims across the stage. Go to the outside of the arena to avoid this. After he does this, he will place his arms on the floor, and a dark current will surround them and he will spawn Creepers. Don’t concentrate your attacks on the Creepers, but take out one whenever you get a chance.

Boss: Axel (1st time)
Difficulty: 3
Strategy: Axel goes easy on you this time, so take advantage of it. He attacks mostly head on, so move around a lot to avoid his attacks. When he attacks, you should circle around him and attack from the side or behind to get some hits in. When he kneels down, this signals he is going to unleash his most powerful attack. He will hit you with a bunch of fire combos, and if you get caught in it, it can cause some major damage. So when he does this try to keep your distance from him and move around constantly.

Boss: Axel (2nd time)
Difficulty: 4
Strategy: Axel doesn’t hold back this time, and he’s no pushover. He’s faster and stronger than before. But luckily, you have two keyblades: The Oathkeeper and Oblivion! This gives you a good taste of power and what to expect later in the game. Essentially, just keep hitting him when he’s open, and let your button mashing do the rest. He still has that annoying combo attack from when you last fought him, but that’s not all. He has an attack that covers the floor in magma, and it slowly depletes your HP. When Axel leaps out of the ring, lock onto him and when he comes out, use the reaction command to counter his dive, then react twice more to unleash an attack that clears the magma and leaves Axel unconscious! The screen fades to white during this attack. Also, make sure not to touch the walls of fire, as they can cause damage to you.  Reaction commands are absolutely key in this fight.  Technically, you can just mash the reaction button the whole match, but if you want to do it the "right" way, watch for reaction commands and chains thereafter when Axel is behind the fire wall, and when you are comboing him.

Boss: Shan Yu
Difficulty: 5
Strategy: This battle can be a little tough unless you know the right moves. Shan Yu’s basic attack at the beginning is a couple basic sword swipes. You can probably jump over it and attack Shan Yu in this way. Also, at the beginning, it would be a good idea to use Mulan’s limit on him. Save Valor form for later in the battle. After you attack him, he has a nasty counterattack where he creates a shockwave, then jumps in the air while spiraling with his sword. If you get caught in this it can cause trouble. So after you get a combo or two in on him, make sure you avoid the shockwave, and keep your distance so you don’t get caught in the spiral. About halfway through the battle, Shan Yu will become surrounded by dark energy. He gets tougher now, so it’s a good time to use Valor Form, for the power and because it has the retaliating slash ability, which is a very good ability for recovering from attacks. You might want to practice using the ability whenever you’re in drive form, because it becomes a major help later in the game. Pepper Shan Yu with attacks and keep him under pressure. If at any time Shan Yu and Sora strike at the same time at each other, they may lock swords. Rapidly press the reaction button to throw Shan Yu across the ring. He also now has a combo attack consisting of about 4 hits, so try to jump over it or avoid it.

Boss: Thresholder and Possessor
Difficulty: 4
Strategy: This fight basically has two different parts to it. First, you must defeat the thresholder and release the possessor from it. One you do that, you can attack the possessor. Note the thresholder has long arms that can hit you from a distance. So stay toward the back of the chamber where his arms can’t reach you. There are two main ways to approach this. You can cast magic from a distance and then wait for it to recharge later, which is the slow way, or you can go up the sides and then attack it directly. This way is a little more dangerous, but takes a lot less time. There are two waves of hook bats and two gargoyles that attack you during the start. Defeat them first and then focus on the thresholder. When you reach him, it is a good idea to attack with valor form, and unleash a lot of combos. If the party’s HP drops a lot, then go back where the thresholder can’t reach you and go back to the first plan: attacking with magic. After you reduce the thresholder’s HP to 0, go up and use the release reaction command to release the possessor. Attack the possessor with combos repeatedly. Eventually, the possessors will posses the door again and you must defeat the thresholder again. Do it the same as you did previously to release him again. Once the possessor’s HP is reduced to 0, you win the battle.

Boss: The Beast
Difficulty: 1
Strategy: This is a fun little battle. Basically, just keep using Cogsworth’s wake up reaction command to daze the Beast, and then attack him with a combo. You shouldn’t try to battle him without reaction commands, because he’s a pretty tough fighter. Once in a while you’ll have to use recharge Cogsworth (about every 4-5 wake up commands). Keep pressing the reaction button to charge. This battle is pretty straightforward overall, and is very easy.

Shadow Stalker/Dark Thorn: 
This battle has two parts, so I’ll break it into two sections

Boss: Shadow Stalker
Difficulty: 5
Strategy: Shadow Stalker attacks by possessing things in the ballroom. If he flies toward the window, rush toward the walls behind the pillars, because columns of energy will cover the ballroom floor. If you’re locked on and he flies up, avoid the center, because he’s going to drop the chandelier and spin it around to attack with lasers. When he stops attacking, rush in to the chandelier and attack it until you get the reaction command release, and use it to release the boss from the chandelier. If the boss disappears into the floor, get away to avoid the ground swallowing Sora up. If he possesses a pillar, the pillars will scrape across the floor. Try to jump in between them to avoid them, and then attack the pillar he has possessed and attack it until you can release him. This is a good time to attack him, as he is near you and vulnerable. Save valor form for his second for though, and use Beasts limit against him.

Boss: Dark Thorn
Difficulty: 5
Strategy: This boss is tougher than SS, so be ready for a tough fight. The boss can turn invisible, so use lock on to counter this. At the beginning it is a good choice to attack with valor form and unleash a lot of combos on him to knock a big chunk out of his HP. There are a few useful reaction commands in this battle. If Sora goes flying through the room, use the slingshot reaction command when you get close to a pillar to counter the throw and cause damage to the boss. If Sora is fighting in the center of the room, you may get a three stage reaction command where Sora uses the chandelier to attack and swings Dark Thorn around the room causing major damage. But make a note that Dark Thorn can also attack with the chandelier. The Beasts limit is good to use here if you have the MP to spare. Reaction commands are key in this fight, so keep a finger next to the reaction button.

Boss: Cerberus
Difficulty: 4
Strategy: This fight is a two on one match with you and Auron versus Cerberus. Auron is very useful in this battle, because of his power, and, most importantly, his limit. Cerberus doesn’t walk around at all; instead he jumps around, which creates shockwaves you should jump over. Cerberus can shoot fireballs from its mouth, so stay to the side of him to avoid this. When its two outer heads rear back, they will attempt to clasp Sora’s hands. If you do this correctly, you can set up a three-stage reaction command: Evade, Jump, and Dog Paddle. This inflicts a lot of damage, and leaves Cerberus unconscious. A good strategy for this fight is to lock on to one of the outer heads and hit it with air combos, to avoid too much danger.

Boss: Demyx (1st time)
Difficulty: 6
Strategy: This is more like an event battle than a fighting battle. To win, you must defeat the 100 forms Demyx summons in 80 seconds. The key is using the wild dance reaction command. This can be initiated by the first couple hits of a combo. Attack one of the water forms, then after two hits, use the wild dance reaction command repeatedly. If you lose the chain of reaction commands, use another combo to start it up again. This battle is pretty tough the first time around, so it will probably take a few tries the first time.

Boss: Pete (Olympus Coliseum)
Difficulty: 4
Strategy: This battle has two parts to it. In the first part, it’s you, Donald, and Goofy versus Pete. The objective is to keep Pete and the Heartless from harming Megara. Use Valor form at the start to take care of the small heartless’ and throw Pete a few punches. Try to keep the fight away from Megara by staying away from her. However, if she’s in trouble, you should help her out. When you attack Pete, he will probably start getting enraged and then create a shockwave, so avoid him when he does this. He will say, “Get outta my way!” when he is about to make a shockwave. Usually he does this after you get a few combos in a row on him. After he finishes, move in to attack him again. Pete may generate a shield and say, “Pete’s invincible!” When he does this you cannot harm him, so wait until he lets the shield down. He will also throw bombs around once in a while; it looks like he’s throwing a bowling ball. After a while, Hercules arrives and rescues Megara, and then joins the fight. In this phase of the battle you have 2 minutes to defeat Pete. With Herc at your side, you can break Pete’s shield by running to Pete and using the pinball reaction command. After you break his shield, this leaves him open for attack. If Pete has a boulder above his head, run to Hercules and make an Aura Guard to defend against it.

Boss: Hydra
Difficulty: 5
Strategy: The Hydra has three forms in this battle. At first, it has only one head. Watch out for its tail, which sweeps back and forth across the arena. When its head gets low enough, hit it with aerial combos. After a while the head will rear back and you can run up to the body and use the vanquish reaction command to cut off its head. The body will now lie limp and three heads will sprout out of the ground. Phil will start screaming at you. Go toward the tail end and hop on Hydra’s back. Use the Phil One-Two reaction command to make Phil throw up a giant urn, then use the Urinator reaction command in the air to smack the urn on Hydra’s back. Do this two more times to make the three heads fall unconscious. Now go to one of the three heads and deplete its small HP bar, then use the vanquish command to cut off that head. Do the same with the other two heads to start the final part of the battle. There is a time limit to defeating the three heads, and if you don’t make it, you will have to repeat it with possibly another head. Now the Hydra has seven heads! He has two magic attacks; in the first one, little black spirits attack you. Fortunately, they can’t turn on a dime, so use this to avoid them. During the other attack, blue pillars of energy erupt in your current position. Keep moving always to counter this. If you are low on HP and MP, look for Megara in one of the corners. She’ll hand over a few HP and drive orbs. When a few of the heads hang low, attack them, and try to hop onto them if you can. You will occasionally get the Pegasus Run reaction command. Use it and press the reaction button throughout to cause major damage. When you deplete Hydra’s HP bars, run up to it and finish it with vanquish.

Boss: Pete (Timeless River)
Difficulty: 4
Strategy: This battle is just plain weird. It’s you, Donald, Goofy, and Pete versus Pete! Past Pete teams up with Sora and company to defeat the present Pete. In this fight you go from stage to stage (all the stages you cleared before in this world). In each stage you must defeat all the heartless from that stage. Once in a while Pete will drop a curtain over the screen and you will end up in the next stage. In this fight Pete tosses bombs and other things around the stage, so keep and eye out for that. He also has the shockwave attack from when you last fought him. Past Pete is a good helper in this battle, but unfortunately you can harm him with your attacks. But it is pretty funny to watch past Pete’s reaction if you hit with an attack accidentally. Keep in mind that at the Building site you can use the Air Slash reaction command to hurt Pete when the scaffolding starts to wobble. At the last two stages, Pete will start shielding himself like in the last battle, so treat it the same way.

Boss: Barbossa
Difficulty: 5.5
Strategy: At the beginning, you will have to find the illuminator heartless and take him out. Look for two glowing eyes on the wall to find him. Barbossa uses his gun to hit Sora, so the retaliating slash ability is valuable for recovering from this. He also has an attack where he lunges forward rapidly, so try to jump over this if you can. Keep in mind you can only harm him in the light, so keep him near the island to keep him in the light. Later on in the battle he will unleash some really nasty combo attacks. But there are a few reaction commands that can save you from this attack. One of them is when Sora steps on Barbossa’s sword and as Barbossa attempts to raise his gun you must use a second reaction command to knock him across the room. After a few of these reaction commands he will get tired, leaving him vulnerable for a bit. Also make note that the illuminator will return once or twice more in the battle.

Boss: Volcanic Lord & Blizzard Lord
Difficulty: 4
Strategy: This isn’t too hard of a fight. The Volcanic Lord and Blizzard Lord will frequently pelt you with fire and ice projectiles as their normal attacks. You can use reflect to deflect these back at them. Each of them has a special reaction command you can use. When the volcanic lord starts bouncing around, use the firagun reaction command to fire the volcanic lord at the blizzard lord and cause damage to both of them. You can also use the blizagun reaction command against the blizzard lord. After dealing some damage to either the fire or blizzard lord, they will split into fiery globes and icy cubes, respectively. Fire and Ice magic work well against the lord of the opposite element.

Boss: Prison Keeper
Difficulty: 4
Strategy: In this battle the Aerial Spiral and Aerial Finish abilities are invaluable, and you may also want to equip the reflect spell. This guy has a lot of HP, so prepare for a long fight. The prison keeper’s attacks can easily be interrupted by an aerial combo, so maintain pressure on him at all times. When he tries to eat one of the children (Lock, Shock or Barrel), you can rush in and use the inside combo reaction command to knock him out for a time. If he eats one of the children, you will hear the name of the child he ate screamed by the two remaining children, and the boss will change color a bit. As far as drives go, valor form is better if you like close quarter fights, but if you’re the kind of person who likes to fight from a distance, Wisdom form is a better choice. This boss has a few magic attacks. If he starts hurling giant fireballs at you, hit them back with your keyblade or use the reflect spell to bounce them back at him and interrupt his attack. After this, rush in to attack. If the boss starts spinning and shooting white energy balls, run up to the top of the hill and attack him to stop the attack. Make use of the hill to avoid his projectile attacks and to hide from him long enough to heal if you need to. The key in this battle is maintaining pressure as often as possible.

Boss: Oogie Boogie
Difficulty: 4
Strategy: This is sort of an event battle. To beat Oogie, you must first knock him out of the control box to start hitting him. To knock him down, you must go up to one of the presents and use the “fore!” reaction command. After hitting him with about 10 presents, he will fall down and you can attack him. This is a good time to use limits or valor form. When he goes back to the control box, he will most likely change positions. To get where he is, go up to the portal in front of you, and wait until it will direct you where you want to go (the reaction box will tell you where the portal will go). Make note that bags with heartless will come down the conveyor belt as well as presents. Also, he has an attack where 5 columns of bolts will come down the conveyor belt. To avoid them just weave in and out between them. Also, watch out for the spikes at the end of the belt, because they can cause you, and even Oogie some damage.

Boss:Banzi, Shenzi, and Ed (1st time)
Difficulty: 5
Strategy: In this battle you must defeat the three hyenas while protecting Timon and Pumbaa. It’s best to fight the battle without locking onto one enemy. Try to concentrate on whoever is attacking Timon and Pumbaa. But if one of them is close to being defeated, then concentrate on that one. They’re all very fast, so if you lose them use lock on to find one. Sometimes it’s a good idea to hang around Timon and Pumbaa and wait until one of the hyenas approaches. You can also use the call over reaction command to call Timon and Pumba over to you.

Boss: Hostile Program
Difficulty: 6
Strategy: This fight can be difficult unless you make good use of opportunities. When you attack the Hostile Program, cluster balls will materialize. When you grab enough cluster balls, you can use the freeze reaction command to freeze the program. Proper timing when using freeze is very important. When you freeze him, you should use Tron’s limit, which will work better than drives. The Hostile Program will spend most of the time against the opposite wall of the heroes. The boss uses a lot of laser attacks, which get progressively more difficult to avoid. When he is in the center, the arms will detach from the body and work independently.

Boss: Demyx (2nd time)
Difficulty: 6
Strategy: Demyx is a bit harder this time. At the beginning of the battle, you must defeat 50 forms in 40 seconds; similar to the first time you fought him. Like before, use the wild dance reaction command to destroy the forms. After you defeat them, he will be vulnerable for a couple of combos. When Demyx strides across the ground, a trail of geysers is left in his path, so try to avoid getting in his way. After this attack he typically uses an attack that sprouts geysers around him. He also has an attack where he makes water bubbles fall from the sky. This attack is difficult to avoid, so defend as best as you can. Similar to this attack, he can shoot a stream of bubbles at you, but this can easily be avoided by jumping out of the way. Once in a while you may get a strange reaction command called showstopper, which makes everyone, including Demyx, pose on the screen together. This doesn’t really do anything, but it does stop battle for a second. When he says, “Dance water, dance!” you will have to defeat 10 of his forms in 10 seconds. When he says, “Come on, kick to the beat!” he will start attacking you and a series of geysers will sprout up in “walls”. When he starts to walk slowly toward you, avoid him, because geyser walls will shoot up in front of him as he approaches.

Boss: Mysterious Enemy
Difficulty: 7
Strategy: Before you enter this battle, stock up Sora with Hi-potions, because it can be a pretty difficult fight. This enemy rarely follows up with his attacks, so take advantage of this. If you circle around him and stay at a semi-close distance, you can avoid his lunging attacks easier. Trinity Limit works well in this battle, try to use it during one of the openings in his attacks.

Boss: Storm Rider
Difficulty: 6
Strategy: This enemy is a very aerial based opponent. So equip abilities like aerial spiral and aerial finish. A good way to attack the boss when it flies high into the air is to use the whirlwind to launch Sora into the air, then land on the Storm Rider’s back. Then, go into Valor form and attack the horns to cause major damage. If you’re on his back and he starts to spin or turn, use the "hold on" reaction command to, in fact, hang on! When he starts to sweep across the arena floor, get under its belly and use a reaction command to knock it out temporarily. When he uses his laser attack, respond by using reflect. If he lands on the bridge, he will create two damaging walls enclosing you in an area with him. He will charge up a powerful beam of light and fire it down the middle of this area. This attack is easily avoided by staying to the side of the enclosed area; however, this is also a good time to attack him.

Difficulty: 8
Strategy: This is a difficult fight, so equip the party with Hi-potions; also, retaliating slash is a great ability to have too. Start out with Beast and Donald in your party. Xaldin is pretty fast and has great range, but some of his attacks, like when he thrusts all his weapons at you at once, leave him vulnerable for attack. With each attack, you have a chance to gain an attack called jump by using the learn reaction command; you can hold more than one jump. Using multiple jumps in a row is a good strategy for knocking him off guard. Once you knock him off guard, this is a good chance to use the Beast’s limit, which will inflict massive damage on Xaldin. If he starts to glow green, try to avoid him, because he will start attacking like crazy, and this can cause you some major damage. If he flies away on his spears and says, “Sora!” go down to the end of the bridge as far as possible (the side furthest away from the castle) to receive minimum damage. If you die in this battle, fear not, because King Mickey is here to help you out! If you die, the screen will fade to white; if you choose “I won’t give up!” you will gain control of King Mickey. When controlling King Mickey, you can hurt the boss, but Mickey can’t kill him by himself. Using the charge reaction command, charge Mickey’s drive gauge until it’s full. Then, use the reaction command to bring Sora back to life. After this the next time you die there is a 75% chance Mickey will appear, and then a 50% from there on.

Boss: Grim Reaper
Difficulty: 6
Strategy: This battle is a little tough, because you must rely heavily on magic and because it’s hard to get the Grim Reaper vulnerable so you can hurt him. You must replace all the cursed medallions to the chest in the center. To knock them loose from him, freeze him with blizzard, then cast thunder on him. When you have all the coins, go to the chest and use the replace reaction command to return the coins to the chest. Now you can attack the Grim Reaper. This is a good time to use Valor or Master Form. If he starts gathering medallions from the chest, run up to him and use the hinder reaction command, and then the Loot launch reaction command to knock loose a lot of his medallions. The retaliation slash helps a lot in this battle to continue your assault him when he knocks you off balance.

Boss: Hades
Difficulty: 5
Strategy: In this fight, the odds are definitely in your favor. It’s a four-on-one with you, Goofy/Donald, Auron, and Hercules versus Hades. At the beginning, you won’t be able to harm him, but after a cut scene where Herc joins the fight, the real battle starts. When Hades is in flame mode, he is invincible. But listen for Hercules to say, “You can’t stop a true hero!” He will create two aura spheres. Use the aura sphere reaction command to hit them at Hades and put him out of flame mode. Once he is vulnerable, use Auron’s limit to inflict major damage. Then, use Valor to restore your MP and cause more damage. When Hades erupts flames around him, find the nearest aura sphere, or run to Hercules and wait for him to make one. This attack puts Hades in flame mode. He can also summon a giant ball of fire that makes hot spots on the ground, so watch out for that.

Boss: Jafar
Difficulty: 5
Strategy: In this battle, it’s a one-on-one match between you and Jafar, and you’re riding on the magic carpet. To hurt Jafar, attack his chest until he becomes immobile. Get close to the end of his "tail" and use the wrap up reaction command then the spin reaction command to knock him out temporarily. Then, attack his head in full force. He has a few nasty attacks to watch out for. One is when he says “Shugalabuga” or something incomprehensible like that. When he says this, fireballs will appear in midair, and small lightning bolts will strike down everywhere. Try to avoid the lightning bolts, but the fireballs shouldn’t be much of a problem. After this attack is a good time to go for the stomach. Another attack that happens a lot during the second phase of battle is when he transports Sora to another dimension, then throws buildings at him. Instead of moving horizontally, try to move vertically to avoid the buildings instead. Attacking the stomach during his attacks isn’t a good idea, because it’s hard to keep attacking while being “tracked” by his attacks. Although a good way to knock him out fast is to get close to him during one of his magic attacks and use reflect.

Boss: Lock, Shock, and Barrel
Difficulty: 5.5
Strategy: The point of this battle is not to defeat the trio, but to trap them in boxes. Use mostly aerial combos to attack them, because most of their attacks are ground-based. You can knock them out for a while by depleting their HP gauge. Before delivering the final blow, however, lead them near a box, to make it easier to trap them. To trap them, run to the opposite side of the box as them and use the reaction command to throw the box on them.

Boss: The Experiment
Difficulty: 6.5
Strategy: This bag of bolts is sort of tough for something that isn’t very well put together. As you attack the experiment, parts of it fall off. Some of the parts regenerate though, and some limbs will actually attack you! The head can also detach and shoot you with lasers. A good way to cause damage is to use Donald’s Duck Flare limit. This can seriously put a dent in the Experiment’s HP. When the Torso spins around like a top, you can’t damage it, but you can use a reaction command to knock it back at the rest of the body, but it still doesn’t damage it. During the second half of the battle, The Experiment uses more counterattacks, so having the retaliating slash ability is essential. Or at least it helps a lot. Using it, you can actually keep attacking, and when he counters, use retaliating slash immediately afterwards and start a new combo! This is a great way to maintain pressure on The Experiment.

Boss: Banzi, Shenzi, and Ed (2nd time)
Difficulty: 7
Strategy: In this battle, you must chase down the three hyenas and defeat all of them. They don’t attack you, but instead they run away from you, and they’re very fast. If you are comfortable using the magnet spell you should use it to pull them in then use Simba’s limit. The dash ability combined with running tackle is also very useful. Eventually you will get the feel for the path you should take and where the best place to catch up with them is. This can take a while, so patience is key here.

Boss: Groundshaker
Difficulty: 7
Strategy: This guy is one gargantuan heartless! The battle starts off with a five-stage reaction command. There are four times you must use fend to avoid taking damage, then one last time to knock the beast out. When he’s knocked out, attack his eyes until he gets up. When he does, run to one of his legs and use the leap reaction command twice to hop on his back. Then start attacking the core of the body. The lasers and the core can knock you off, so having the retaliation slash is very useful. Simba’s limit causes massive damage, so use it whenever you can against the core. When the core disappears and the beast knocks you off, flames will start chasing you on the ground. Avoid them using the dash ability. When the Groundshaker starts leaping across the gorge and has flames on him, wait for him to hop back. After he stops stomping around, run under him and wait for him to start stomping again. Use the fend reaction when it appears to begin the five-stage reaction commands again. The battle will repeat with the same pattern from the start until you defeat it.

Boss: Sark and MCP
Difficulty: 5
Strategy: The battle starts out just fighting against Sark, and some Strafers. He isn't too tough. The only thing to watch out for is when he says "Data transfer". This means he is going to teleport. Once you defeat Sark, the MCP powers him up. He will become giant, and you will have to attack his ankles until he kneels down. Then you must attack his head. It usually takes about 2 or 3 times attacking Sark's head to knock him out temporarily. He will say "Data error" when he is knocked out. When he's out cold, attack the walls surrounding the MCP. When one of the panels breaks, run to the opening and use the delete reaction command to deplete the MCP's HP. Tron needs to be around for this attack. Rapidly press the reaction button during this time until the MCP attacks you with lasers. Eventually Sark wakes up. He says "Corrupt data restored" when he wakes up. After this you will have to knock him out again, then start attacking the MCP again. As the MCP's HP drops, his attacks get more aggressive. Some of his attacks go over Sora's head, but some you'll have to jump over.

Boss: Xigbar
Difficulty: 7
Strategy: The battle starts off with Xigbar shooting at Sora, Donald, and Goofy from a distance, from his point of view. The three rotating dots try to lock onto you. You can try to avoid it, but if you do, avoid the top part of the arena; it makes him easier to lock onto you there. You could always stand still though, and use the reaction command three times to knock his shots back at him. Once you do this a couple times, he will come back to the stage. He teleports around the stage while floating motionless in the air. Using limits here is a good way to cause damage, because it keeps him in one place and lets you attack at the same time, as long as you use a limit like Donald’s Duck Flare. If you have Final Form, it’s also a great choice of attack, or if you haven’t unlocked it yet, now is a good time to do so. The time to attack is when he says "Reload", which leaves him unguarded. During one of his attacks, blue bullets will fly around the room. You can use the Warp Snipe reaction command to knock them back at Xigbar. He has two attacks to watch out for. One is when he turns blue and shoots multiple shots at Sora. The other attack is when he isolates you in the center part of the stage and bombards you with shots from all directions. Use the reflect spell to counter both these attacks.

Boss: Luxord
Difficulty: 7
Strategy: This battle introduces a new factor: time. Instead of having HP, the battle is fought against time. At the beginning, Luxord will say, “Whoever’s time runs out first is the loser.” Basically, both time gauges are HP bars that deplete at the same rate. Attacking Luxord makes his time bar deplete little by little. However, if your HP reaches zero, you can still lose. Luxord may change Sora into a card or a die. When he does this, lock onto him and hop toward him, then use the attack button to attack him and interrupt his attacks. Keep doing this until Sora changes back to apply pressure. When he says, “Cards” he will turn himself into a card and mix himself with other cards. You can stop this move by using a finishing move from a combo before he transforms. Find his card and use the flip reaction command to transform him back and make him vulnerable. When he spreads cards across the floor, use the Glide ability from Final Form to float across the arena. You can also use the "begin game" reaction command. When you use this you must get 4 “O” commands to avoid being transformed into a card or a die again.

Boss: Saïx
Difficulty: 6.5
Strategy: Saïx’s specialty is going berserk. He will say “Moonlight shine down” and then go berserk. Before this his berserk gauge will fill slowly and he will hold his hands up in the air leaving him vulnerable His main attack when in berserk is doing two jump attacks, then a finishing jump. When he does this, he will say, “Duh! Duh! Be gone!” In the upper left hand corner there is a gauge, similar to limit gauges, which shows how much longer his berserk status will last. If you want to avoid him until it’s over, use the aerial dodge and glide abilities from Master and Final forms, if you have them. But, unfortunately for him, Saïx shows the same flaws as normal berserkers. When he’s attacking, he leaves his weapons in his wake. You can grab one of them using the berserk reaction command, then go to Saïx and use the Eclipse and Magna Storm reaction commands when prompted; this knocks him out of his berserker status, but it doesn’t hurt him. It can hurt him though if you do it to him when he's not berserk. The only tricky part about this attack is that Saïx's weapons don’t work like Berserker's cursed hammers. You have to aim the attack yourself, instead of enemies automatically being drawn in to the attack. After this he is vulnerable, so attack now. Again, this is a good time to try and activate the Final form if you don’t have it yet (Plus, if you defeat one of the Organization members while in final form, it counts as a Nobody kill). When Saïx isn’t berserk, he’s vulnerable to attack, so use limits or drives to cause a lot of damage.

Boss: Xemnas (1st time)
Difficulty: 5
Strategy: This is a one on one battle between Sora and Xemnas. He isn’t too hard though. He only has two or three attacks really. At the beginning he will surround himself in darkness and try to zap you and create an energy shield that slowly drains your energy. However, you can use the glide agility to stay away from him, but I’m pretty sure the battle won’t progress until he gets you. When he zaps you, run up the stairs on Memory’s Skyscraper and use the Face Down reaction command. This frees Sora from the energy shield. Then, Sora will see Xemnas on top of the skyscraper and run up the side (sort of like in Another Side, Another Story), when they meet, use the clash reaction command to knock Xemnas down. If you don’t execute the clash command, nothing will really happen. After this you can attack Xemnas with combos. One of his attacks now is to create a shield in front of him. If you have the glide ability, you can circle around him and attack him from behind, or just run around the shield. His other attack can be dangerous. He spawns two light beams from his hands (they’re sort of like light sabers from Star Wars). Then he will go into a flurry of attacks, moving very fast. Using the glide ability is very useful here. But even with that, Xemnas will still stay close on your tail. Between his attacks is the best time to attack him. Keep in mind that you can get in several combos in a row on him at a time.

These battles are not classified as mainstream boss battles, but they're not normal battles either. I'll use the same format as in the first section.

Battle:1000 Heartless Challenge
Strategy: This battle may seem daunting at first, but it's actually not that hard as long as you use reaction commands. Normal attacks are not really needed in this battle. About 90% of the enemies are Armored Knight heartless. The rest are Surveillance Robot heartless. The key is using the reaction commands. Luckily, there are enough Armored Knight heartless to where at least one of them triggers the Rising Sun reaction command most of the time. Also, Surveillance Robots sometimes trigger the Snag reaction command. Use it to grab them, then use Sparkle Ray to really do some damage! Just use these throughout the battle.

Boss: Leon and Yuffie
Strategy: Yuffie is weaker than Leon, and she gets harder if you defeat Leon first. She teleports a lot, which makes her hard to hit. The easy way to defeat her is to use one of Donald's limits, then go in and attack her simultaneously. Watch out for when she levitates a weapon, this means she is going to attack with a giant Shuriken. Once Yuffie is eliminated, it's time for Leon. His attacks aren't too difficult to avoid, and they're all mostly close range attacks. He pauses between attacks, which makes him very vulnerable. When he says "Here it goes", he will shoot fire at you.

Strategy: Yes, Sephy is back, and he is stronger than ever! He has 3000 HP, which is 14 full bars! Once again I'm going to split this strategy into sections.

To Prepare
This is a one on one battle with Sora alone. First, it helps to be at a high level, somewhere around 80-90. This is so you have high defense and a lot of AP to equip all the necessary abilities. You should also get every Get Bonus in the game (besides the one from winning this battle). As far as abilities go, there are a lot you should have. For action, the essential abilities are: Retaliating Slash, Slapshot, Slide Dash, Guard Break, Explosion, Aerial Spiral, Aerial Finish, and Trinity Limit if you so desire. For growth, the essential abilities are High Jump lvl.3, Aerial Dodge lvl.3, and Glide lvl.3. For Support, the essential abilities are Scan, 1 or 2 of each Combo Plus, Combo Boost, Leaf Bracer, Combination Boost(only if you equipped Trinity Limit), Second Chance, Once More, MP Rage, MP haste, and Item Boost. Some accessories you can equip to increase your AP for all these abilities are the Cosmic Ring, Cosmic Arts, and Expert's Ring. For items, you should have at least 3 elixirs, and the rest of his slots should have Hi-Potions. It's also important you have Ultima Weapon, for the MP Hastara ability.

Phase one of the battle
At the beginning, he'll immediately say "That's enough". You will have to be quick to use the Block reaction command to block his attack, or it will cause major damage. After this attack, he will be left unguarded. So hit him with a combo, then he'll disappear. When he reappears briefly he will knock you into the air. Let him do this, then use the square button to use retaliating slash when he reappears again to get the jump on him and hit him with a combo. The attack to look out for is when he goes into a flurry of combos on the ground. He usually does this if you avoid the previous attack (both when he tries to knock you up in the air and hit you in the air). If he does do this, Glide away from him until he stops. He will occasionally use the attack where he says "That's enough" again, so be on your toes. Keep in mind that a reflect spell can also stop this attack. But blocking it is much easier.

Phase 2 of the battle
After losing about 3-4 of his HP bars, Sephiroth starts to get serious. He still will use the same attacks as before, but now he has a new attack. He will spawn dark energy balls around Sora. He says "Give in to the darkness" prior to this attack. Use a reflect spell to take care of this attack. He also starts using his most powerful attack. He will teleport away from you, and slowly rise into the air, and he will say "Descend Heartless Angel". Use Glide and aerial spiral to knock him out of his stance. If you can't, Sora will be left with 1 HP and 0 MP. Another new attack he has is when columns of fire erupt around him. This pulls Sora toward the attack, and can be fought by just running away, but it's better to Glide away, just to be safe.

Final Phase of the battle
Once Sephiroth gets down to 5-6 HP bars, he will start to glow red and run around the arena really fast. He's really hard to catch, even with Glide. After running around he usually does one of two things after teleporting. He'll either go into a flurry of ground combos, or try to use Descend Heartless Angel. He'll also ramp it up and use a lot of the attacks he used earlier in the battle. Plus, he has yet another new attack. He will teleport off the stage and rain comets down upon you, and these things aren't like Donald's comets, they're huge! Use a double jump and glide around the arena to avoid the comets. If you get hit by one, you're in trouble. Try and use retaliating slash, then a quick aerial dodge backward, then glide to break the cycle of getting hit over and over again. A great way to attack throughout the battle is using the berserk command. This command replaces the attack command during MP charge. So, after using Curaga or Trinity Limit, you can attack him hit-after-hit without stopping! He will eventually escape though. But you can easily take off one HP bar from him with this. A good way to cause a lot of damage is to use Trinity Limit, then follow up with long stringed combos during MP charge. This battle takes more than one try, so stay patient and try to get a feel for dodging his attacks and his various attack patterns. 

After the battle
After you beat Sephiroth, he tells you only Cloud can truely defeat him. Go tell Cloud about Sephiroth and then go back to watch a fight scene between the two of them. Also, make sure to keep a separate save file, because you won't be able to fight Sephiroth anymore on that file after the fight scene, unlike in Kingdom Hearts 1 when you could fight him over and over.