When I played KH2, I found the area to fight sephiroth(late in the game). The keyblade that I had the easiest time beating him with(on each difficulty) was the Fatal Crest, Or at least I think that was it(correct me if I'm wrong). But theat keyblade gave me the ability to have an unending combo. That didn't become the case every time, I had to fix my abilities into a certain way as well, make sure that cure was a quick spell, and stock up a lot of elixers in my pocket. the triangle button was essential, and the air recovery helped me in the fight quite a bit as well. Dodge roll was one of the abilities as well, that made the meteor attack less destructive to my health, along with that move where you deflect their attack, I was barely ever touched during that attack. Defence raising abilities were also a very big part of winning with ease, can't kill you, can't win...get it?
Any way, I also tried stupid things like trying to go into the fight while still in valor form(doesn't work) or even constantly pressing the triangle button(also doesn't work). You have to have a really good strategy, or a really high level helps, but any way Victory is assured if you play the battle smart, or just kill him before he has a chance to kill you. *unconvincing smile*
That is all