Just did this as a 110 chanter, to run an alt cleric through. Mega Lavabomb parsed at 7.35M hp, took 60s to kill, with me punting back each time banishment refreshed.
Cast Dissident Reinforcement just before previous mob died, then triggered Improved Twincast, Illusion of Grandeur, Calculated Insanity and Third Spire, and threw everything I had at it