1) Level up. I would suggest 100, but if you don't have time...
2) Max out all Growth abilities (High Jump, Glide, etc.) by levelling up your Drives.
3) Synthesize yourself as many Elixers as you can carry, and take them.
4) Make sure you have the Ultima Weapon.
5) Synthesize as many status-boosting items as you can, and use them on Sora. (Power Boost, Defense Boost, etc.)
Do those five things to prepare yourself, and you should be fine. Reaction commands are key in the battle if you don't want to get instant-killed. Also, if yo don't have the abilities Once More and Second Chance equipped, go erase your game file now. Those two abilities, along with Explosion, are the BEST abilities in the game hands down. As you progress through the battle, he uses increasingly powerful attacks. Be prepared to face off against his old standby, Meteor. That is all I shall tell you for now. If you still need help, contact me personally with detailed game info and I will instruct you further. In any case, good luck!