A wise person would make an audio trigger for Crystalline Expuntis. This spell is a 1200/tick mana drain AND a 600/tick endurance drain. The resist check is -400 chromatic, but I got hit pretty often with 635 resists across the board (minus corruption).
Use the following for your audio trigger: "You are weakened by a blast of crystalline energy" I also made a hotkey to inform my group I needed a cure.
Also of note, this spell is directional. Face the exterminator away from the group to avoid nailing the entire group.