Since everyone else has already said their way of deafeating Sephiroth, I shall sya mine.

Have charas around level 60, Have KoTR, and Omnislash. I didn't mime KoTR once during teh entire battle. I have been leveing my charas since then and they are now 80. Sephiroth is so freaking easy it makes me cry. Also Omnislash took me a whole 45 minuets to get.