If you grinded all of your characters to Level 99 and are planning to fight Sephiroth without any cheap tactics, then be prepared for a long fight. When you fight Bizarro Sephiroth, his body gets a 5000 HP bonus for each Level 99 party member, while his core gets a 1750 HP boost for the same. His other body parts also get a small HP boost for each Level 99 character, but these bonuses are nothing when you compare them to his second form.