If Andarial, presumably the issue is poison resistance, depending on your merc's level and poison res. You could use Venom Ward when fighting andarial.
You can use antidote potions too, to stop poison and give +10 max poison res and +50 poison res. Probably easier than swapping out armor.
A merc belonging to a Fire Sorc who 4-5-shots her before the merc gets there :]
Maybe Fortitude could help by boosting his defense. Or alternatively something like The Gladiator's Bane for damage reduction. But I'm just speculating.
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ceving
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