In my experience, you don't. If you get hit from a flamestrike, you die.
SPOILER
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If you don't have TOTSC and therefore, Otiluke's Resilient Sphere, then you're in for some fun. Use haste on your party first and equip one of your fighters with the Bastard Sword +1, +3 vs. Shapeshifters. Have any mages cast flame arrow (if they're not invokers or diviners) and magic missle (again if they can). DO NOT use Fireball or Lightning Bolt. Chances are that you'll hit the Dukes if you use them. I was able to save both of the dukes myself, but it took me 5 or 6 tries. Good Luck!
-Jafin
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I am Jafin, arch-mage of the HADB clan!