Being related to Genomoth, Sephiroth has a lot of health and defence, so be prepared for a long battle.
Use Divine Fist a lot; due to its large health and defense, normal attacks will not do much unless they're critical.
Sephiroth is generally Aqua-Elemnet, but can also use Gale-Element magical attacks. In short- zap it with Absolute Zero but be prepared for it to counter with Ruwach. If possible avoid magic, although you'll be at a great disadvantage if you aren't beating it Element-wise.