Here's a boss guide:
This guy is really, really, tough. Get ready to spend a lot of time looking at the Game Over screen.
He has quite a few attacks:
1. A short melee combo (This is blockable)
2. An extended melee combo (Blockable as well)
3. He turns blue and does a short charge towards you. (Blockable)
4. He turns blue and charges across the room 3 to 4 times. (Blockable)
5. A Tornado attack that flings all your commands on the ground
6. An X-Blade attack (This is the most easily avoidable and slowest attack)
7. A 'Pendulum' attack. This is where he jumps up in the air and throws his weapon at you, then pulls you in and follows up with one of the attacks listed above. There is a chance that he will use the X-Blade and you'll be able to survive, but most of the time if you get hit by this attack you will die.
8. He summons a bunch of orbs and they shoot missiles at you 5 times. This is the hardest attack to dodge, and when this happens you should be mashing square as fast as you can. If you attack him while this is happening, he will go back to his regular routine while the orbs are shooting at you, so DON'T. Sometimes a few of the orbs will hang around; when you stop moving they will shoot at you. The best way to handle this is to get far away from MF and get a Curaga spell ready.
9. He summons 4 clones of himself and they all chase you. They all share the same attacks and if you let them hit you it can be deadly. The best way to deal with them is to use Fire Surge or Thunder Surge.
10. He cloaks himself in a field of fire, teleports behind you and charges at you. This happens 6 times.
11. He casts stop on you and rewinds time to heal himself.
12. He can block your attacks with Restore Barrier and heal himself. This means if you use shotlocks, keep the number under 5 or he will likely block the rest of your attack.
13. He summons a meteor that crashes into the ground.
14. He casts a spell similar to Mega Flare.
15. He uses a spell called Doom (the white lines) and you have to mash X to get out of it.
16. When he gets to about half health, he will start turning invisible. He will continue using all of the attacks above during this time, and he is able to use multiple attacks at once, for example he can cast Tornado while cloaked in fire.
Him having all those attacks sounds quite intimidating, and it is, but the good thing is most of these attacks can be avoided by simply mashing square. If you're using dodge roll/dash/cartwheel 100% of the time spent not attacking, you will not get hit. This means start practicing your button mashing... if you're good at Mario Party this will be easy for you. Your thumb is going to get a workout during this fight.
I've seen this fight done by someone using nothing but Fire Surge to do damage, and this works well, but when I first started fighting this guy I came up with the Mine Square strategy. Mines do not hurt the Mysterious Figure, but they DO fill your commands gauge. If you can get him to run into enough mines, you can unleash a finish command on him. I wouldn't recommend trying this strategy unless you have your character's 'Ultimate' finisher. If your command meter starts going down you can use Zero Graviga, with its long reach, to give it a boost during one of MF's safer attacks.
Mines will also make MF and his clones jump over them, and you can use this to your advantage. It will give you a bit of control over the fight if you can make them jump over the mines.
You can also wait for the right moment and use a quick 5-or-below hit Shotlock. If you're able to fully charge your shotlock, there's a high chance MF will just block half of your attack and heal himself to full. That's not good.
If MF summons his clones and you have mines down, try to make them run into the mines and use your finisher. If he summons clones and you don't have mines down, you need to use Fire Surge immediately or you'll probably get hit.
Once you get MF to about half health, he'll start turning invisible. This is where all your thumb exercises pay off (You did do those right?). Since he is invisible, you can't target him so the only thing you can do is dodge constantly until he turns visible again. If you can mash square while trying to lock on at the same time, do it, but that can be tough.
After he starts doing the invisible thing, mines and Zero Graviga really aren't that useful. You'll have to rely on Fire Surge to do most of the damage from this point on.
Easier with Ven
Keyblade: Void Gear
or potions,elixirs on Curaga Slots