Level up. :|
lol, if that's the battle that takes place inside Sin, all you gotta do is follow the magic pattern, use a lot of Hastega, use Yuna's NulBlaze, NulShock, NulTide, NulFrost, and right before he uses Total Anihilation just Summon an Aeon and use shield.
It's been a while since I played that battle and I really don't remember very well the exact technique to kill that fool. But what I said should work.
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