I beat Sephiroth in KH 2 at lvl 60, and on my second try lvl 42, here's what I did for lvl 60
It's easier at lvl 60 if you have beserk. Equip Oblivion
For your magic have curaga
Make shure beserk is equiped!!!!
Oblivion will make magic restore itself faster
Also you need High jump lvl 2 and double jump lvl 2 from valour and master form.
you need the high jump to get sephiroth when he uses Heartless Angel and double jump for when he uses meteor to hit the whole stage
Lvl 42, I did the same thing without beserk, just hit one at a time. At lvl 51, make shure you max out your form abilities