One of the first things he did when I fought him (and he was in dragon form) was that he cast "Protection from Magic Weapons". I simply had Aerie cast Pierce Magic and Imoen cast Pierce Shield and either one or the other seemed to work. I think that Khelben's warding whip will also work.
He was a real bastard though! He definitely put up a challenging fight for me! Kept casting Death Spell on my summoned fodder and instantly dispelling them. Grrrr!
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Now where did I leave that doughnut?!