To beat Sephiroth, you should be around level 80. It would be smart to have Lv. 7 glide and high jump. He is the most hardest boss in KH1 and KH2. With 3000 hp, this won't be a walk in the park. Equipt healing potions and use spells often. The second chance ability is a must. If you beat him, you get the strongest keyblade in the game, good luck, you'll need it.