Here's another tip: Enter from the rear and surprise-attack him.
You can do this from the roof of De'Arnise. It'll place your party directly behind him; enough for your mages to get off a magic missile and your fighter to give him a whack before he fires off his contingency.
Also: have your mages memorize "Breach", "Spell Thrust", "Remove Magic", and/or "Secret Word" to bring down his defenses while your fighters whale on him.
Using this strategy, I'm generally able to pull him down without a hitch. [img]smile.gif[/img]
Cheers,
-Sazerac