It doesn't matter which weapon or which level you have in order to beat Sephiroth. I've fought him both with my levels in the 70's and on level 100, and the battle is very similar either way. I use either Lion Heart or Oblivion in that battle. I think the game has enough colors for 5 health bars and Sephiroth has 6 health bars, so you'll have to take one bar away from him so that the visible bars start dropping. I always let Sephiroth his Sora with his fire barriers because Sora has MP Rage and MP Haste abilities equipped, so being hit by the fire barriers lets me regain MP to heal Sora with Curaga.