For this fight you will need: Knights of the Round, Zero Bahumut, and any other summons. Cure 3, Wall, "Mime", DeBarrier, Remedies and at least three Megaexlir's.

Start out by casting Wall on your party, then cast DeBarrier to take away Sephiroth's wall. After this has been done throw everything you've got at him, use Mime to cause major 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 alright, watch out for his Heartless Angel that will not kill you, but bring your health down to 1. Then use Megaexlirs. Keep up the endurance and Sephiroth shouldn't be to hard to beat.