To defeat Sepheroth, you have to have Elixers, or alot of Hi-Potion's and to keep in mind about his first attack (The one where he slashes alot), When the reaction button comes up, press it. Then, when he does that spell where you're only left with 1 Hp, hit him when he's saying it. It's best to go into beserk mode, by depliting, or drain your Mp bar. After that, you should be able to do countless combos until your Mp bar comes back. But after your done with all that, you're gonna have to be careful with the rest of his attacks. Well, I hope this helps you out with defeating Sephiroth.