How do you defeat sephiroth in kingdom hearts 2 ps2?

Its sort of simple really. You see, some really 'skill people' would use the Ultima Weapon. Some really cheatful people would use Codebreaker. But if you want to defeat him only with the Oblivion Keyblade, best thing to do is just level up.
Go to the peak in the Pride Lands on Pride Rock, where you defeated Scar. There you will see 1000 Heartless, so just defeat them all. Go back into the King's Den, then go back into the Peak, and you will see those 1000 Heartless AGAIN. Defeat them all again, and do the same thing over, over, over, over and over again. This way, you will gradually level up. When you get to level 97, 98, or 99, (there is no lv.100).
Go to where you TRIED to beat that mean Sephoroth, and there, you must equip all Ariel abilities except Ariel recovery, the counterattack ability, beserk charge, once more, second chance, scan, leaf bracer. Scan is only to see how much health your enemy has left. Now, you also must equip these shortcuts: X=Curaga Square=Reflega. Firaga and Blizzaga won't damage him. Make sure you equip this somewhere else, because if you don't win and you equipped these abilities where you defeat Sephoroth, you'll have to equip these abilities AGAIN. Have fun and good luck