How do you defeat sefiroth on Kingdom of Hearts 2

Your best bet is to be at a decently high level. Using abilites that you gain from your drive forms such as glide, quick run, and high jump are useful as well. Have the strongest weapon that you can equiped. Having Elixirs in your item slots is a lifesaver if you get hit with descend heartless angel. But it's best if you stop him from using the attack if you can. Be sure not to miss any reaction commands as they can help negate the large amount of damage that Sephiroth can cause with his Flash attack. Trinity Limit can be used to do huge amounts of damage to Sephiroth. Other than that, Reflega can help immensely if you use it correctly. And the second chance ability can be a literal life saver.