Well, I like to hit him with my swords and cast a few spells. That ussualy does it.
A bit more seriously, you can try doing a search of the forum, there have been a few threads on beating him, or if you like, you can post your party's levels and such, and we can try to come up with a detailed strategy.
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If I asked, would you answer? Its your problem. Its a deep, deep problem. I have no way to ask about that... I have no elegant way of stepping into your heart without tracking in filth. So I will wait. Someday, when you want to tell me, tell me then. -Bleach