How to beat Sephiroth in Kingdom Hearts??

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