here's my take: if you have some charm/fear/confusion rings, helms, or potions, use those and focus on Malavon's "double" and "real" exclusively... he is going to keep pounding your party with spells (which require savings throws that are a lot less "random" than chopping up an umberhulk) once he is gone, you can move your party to an area that is not in his most recent gas/fog spell range, and keep the hulks in that gas zone.
see-- the gas spells that Malavon casts also affects the umbers... so, they'll be weaker by the time you get around to them, but you'll have dusted off the wizard already, so no more nasty spells from his wasted butt.
the iron golemns are strong, but slow. a "strength" potion/spell, or "haste" spell will help to pound them into rust.
-H/M-
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