How to beat Sephiroth in Kingdom Hearts??

u are talking about Kingdom Hearts I and not II right?I will give u both answers.First u must have plenty of Elixirs of Hi Potions with u because the "Descend Heartless Angel" verplaats leaves u with 1 HP and no magic!!!No magic means no Cure and no Cure means no meer HP.You can stop his spell before he casts it door hitting him!In Kingdom Hearts II u need the Aerial Dodge ability (you get that after leveling Master Form [Master Form levels up when u get Drive Orbs]) to reach him.In Kingdom Hearts I u must be lucky enough of Sephiroth be fool enough to teleport behind u before he cast the spell.If he casts the spell go away from him as far as u can and give a Hi Potion to yourself.Better have it in the shortcut menu.Hope I helped ;)