Entry location: Top of Peach's Castle
This is it, the final battle! Dark Bowser is definitely a tough one, so here's a few hints!
1. When he throws a giant boulder up into the air and shoots it with a fireball, flaming rocks will come falling down above Bowser. They fall in 3 columns: Left, right, and middle. For those in the middle, just use Shell Defense to bounce 'em off. When one falls in the right column, however, punch it in order to launch it into Dark Bowser's mouth. When you do, Dark Bowser's attack, Dark Flame, gets cut off and ends up hurting him instead.
2. When put in a cage of dark energy, you have to punch! Fast! I mean it! While Bowser is punching repeatedly, Dark Bowser is charging up a massive attack. You have to punch the lock off of your cage before that attack hits you! Just keep punching, and it falls after about 13 or 14 punches.
3. When Dark Bowser charges up his punch, as soon as he stops charging, you punch too! When your fists collide, Dark Bowser'll jump in the air and try to hit you from above. Here's a combo opportunity! Use Shell Defense to block his fall, then punch him as he lands!
4. After causing a lot of damage, you finally knock Dark Bowser out! But the Dark Star isn't defeated so easily! Fawful uses his power to revive and even grow Dark Bowser! When he's huge, Dark Bowser is immune to punches. When he gets that big, he'll launch a barrage of Dark Bowser Minions at you and knock you far back. Bowser will begin stomping back, but Dark Bowser's minions will be coming at you! If they come on the ground or close to it, just punch 'em. If they're higher up (or if there's a Thwomp above you) just duck. Remember, once you've cleared his minions, Dark Bowser himself will drop out of the sky! Just duck his first one, then duck his second attack. Back to the battle!
5. Once it's your turn when Dark Bowser is huge, use your Flame Attack on him! Caught off-guard, Fawful will fall out of his mouth. Use the Vacuum Block to suck Fawful in, where Mario and Luigi are waiting!
6. In order to make the Dark Star vulnerable, you need to take out Fawful's three legs. To do that, you have to take out his glasses so he can't see what you're doing. The best way to do that is to use Special Attacks! My favorite is the Magic Window attack, because those that can keep up can cause massive damage in one go! Once his glasses are broken, go for his legs! When they collapse, the Dark Star is vulnerable to attack.
7. Getting the Dark Star with special attacks is also a very good idea, since normal attacks aren't very effective. After awhile, the Dark Star pulls Fawful back into Dark Bowser. Just keep up Bowser's attacks at this point, and then suck him back in! Keep it up, and soon, the Dark Star will fall!
8. Don't think the Dark Star falling ends the battle! Bowser's ready for some SERIOUS payback! Once the Dark Star is beaten by Mario and Luigi, Bowser starts his punches! Of course, you will be throwing those punches! Keep up your power, and Bowser ends with one hard, heavy, VERY satisfying haymaker! Congrats! You won!