Bring them back to life
Once the ads get to very low health they will start to cast Dark Martyrdom. If they succeed they die in an AOE blaze of glory. In addition, if they succeed Lady Deathwhisper will bring them back to life as reanimated versions of themselves. These reanimated versions can only be harmed by Forsaken World Gold damage opposite to what they cause. Meaning the casters can only be harmed by physical damage and the melee only by magic.