Required abilities: Strike Raid, High Jump, Glide, second chance. Required spells: Curaga. Okay, first sephiroth will do slashing combos and walls of flame. When he does the wall of flame jump back.If he does the combo, just keep your distance from him. then counter with strike raid. Keep doing that until you run out of MP.When you run out of MP, get yourself hit. After a while, He'll say "Sin Harvest." Superglide up to him and attack.That should stop the spell. If you get hit, the spell reduces your HP and MP to 1. If the spell hits you, use an elixer. After that, he'll just attack with dark balls and a spinning attack. Just keep using strike raid. Keep repeating this and you'll beat him eventually.