You need a good thief. Make the thief invisible and then find and disarm the traps that summon the Doom Guards. If one of the Doom Guards arrives, keep him busy with skeletons or summoned critters while the rest of your party kill him with ranged weapons. If you get two Doom Guards you are in deep trouble.
If you can hurt them, leave, rest, and return it does help. They don't seem to heal.
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