you can't. It's all you when you fight Sephiroth in KH2
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He is made out of matter, and as long as you believe hard enough, your attacks will be able to damage him. The in-game mechanic to support this is hammering on the select button as you attack Sephiroth. If you do not do any damage, you are not pressing fast enough.
In what game? FF7? Crisis Core? Dissidia? Kingdom Hearts? He's in more than one game, so the best answer I can give you is look up an IGN walkthrough or something.
In Kingdom Hearts 2 there are 2 kinds of "Special" Keyblades. The first kind is the Ultima weapon which is a Synthesis item. The recipe can be found in the Mansion at Twilight Town. The second is the Fenrir Keyblade. This Keyblade is obtained after defeating Sephiroth in Hollow Bastion or Radient Gardens. He can be found past the spot called "Crystal Fissure"
no, Kingdom Hearts was made for the PS2 only, and there is a new KH game coming out soon for the DS
By attacking her.