Have the following: Knights of the Round, Zero Bahumut, and any other summons. Cure 3, Wall, "Mime", DeBarrier, Remedies and at least three Megaexlir's. Begin by casting Wall on your party, then cast DeBarrier to take away Sephiroth's wall. After this, throw everything you've got at him and use Mime to inflict serious damage. During the fight, Sephiroth will levitate higher, signaling he's preparing to cast Super Nova. If you are above 1500 energy you should be ok. Be careful forHeartless Angel, which brings your health down to 1. Then, use Megaexlirs. Keep up the endurance and Sephiroth will be defeated.