Don't cast Icestorm. He's 100% resistant to it. Same with most of the moon spells. All sun magic is effective. Right at the start of the fight I like to cast Summon Fiend as many times as I can. I was able to get 8 demons right from the start. Lightning isn't too effective as I remember. Don't use Vine Magic. They don't work either. Burning Haze is a good way to deal with the Trees o' the Grove that he summons. After you get him into the yellow, and if there aren't of his little buddies there, just start hacking away at him. Oh, Meteor and Meteorstorm work well too.
Good luck!
-Jafin
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I may look like an old man now, but just you wait until I take off my fake beard and wig and show you Tiltowait!