Actually, there is a good way to beat him. Use the skill that lets sora throw these spinning air thingies (I think that ws strike raid). Equip lots of MP rages (priority!), have preferably curaga already, that skill, and maybe elixers for emergency.
Everytime you get hit your MP will go almost full so that'll give you lots of chances to heal and do the skill. While you do the skill, he wont be able to hurt you no matter what. You would even want to intentionally get hit just to recharge MP. His life bar wont move for a number of hits. I think it must be invisible HP or something like that. But it will eventually go down. Lock on him. If you're not sure where he is it's dangerous and you need to lock on him in order to use the move.
He has this move that'll put your hp REALLY DOWN. That's when you use your elixers. Sora's slow in using it so use items, so when sephi is stating his move use it. If you time it right, your hp will instantly be full just right after it was almost empty. It'll really save you from dying when he hits you right after he does the move. Then for the rest when he does the rock thing, maybe you should rely on healing until it's done. try evading if you can. I wasn't really sure how to evade that though so I cant help with that. That's all I can give. I hope it'll be useful to you.
I wasn't able to beat him with Ultima since I was too lazy getting the goos but I guess that'll be really good if you use ultima. You can do it with lvl 63 or a bit lesser than that.