Obsidian Destroyer

Mob Details
Has an AOE Drain Mana. If their mana tops off, they blow up doing about 6k AOE nature damage (range unknown).

Strategy
Quite simple: Let the tank get some aggro on the Obsidian Destroyer before starting dps, but classes with Drain Mana abilities should start mana draining as soon as the Obsidian Destroyer is initiated. Just have your druids heal the MT (who should not take that much damage) and the Obsidian Destroyer goes down easily.