First equip your best keyblade, (i waited till i got oblivion), then find the heartless that drops elixers and get enough to fill soras item slots. Then start the battle. Normally he goes and starts with a deadly hit that your reaction should easily counter. After that lock-on and use your trinity limit. You should do a big combo that would rid him of a bar or maybe more. After your limit finishes both of you should be in the air. Start attacking, about 4-5 hits and he will regain balance. Then you have to wait for an opening. As soon as hes open do a combo till he teleports away. Then you might need an to use an elixer. By repeating this process enough you should kill him. Remember, reaction commands and dodging with aerial recovers are very good use.