-When facing Sephiroth the first time, just use Knights of the Round to immediately win. Even though one of his five parts does not take any damage, he cannot live without it. When you battle the second Sephiroth, use Knights of the Round, then Mime Knights of the Round. Now have another character use Bahamut Zero. This should kill him. When you fight the third Sephiroth, use Omnislash.(Cloud's Level 4 Limit). Even if you only have the Level 3 Limits, the game allows you to use it. After this, Sephiroth will die for the final time.
-For Sephiroth Bizzaro, use Knights of the Round. For Safer Sephiroth, use De-Barrier then use Knights Of The Round. On the final form, your limit should fill allowing you to use OmniSlash.
-When facing Sephiroth at the end of the game with only Cloud, if he is level 99 and have Omnislash, one attack will kill him.