We tried this again this year with about 20 characters, mostly around level 110.
At about 75%, several Fist of Krasnok spawned and that finished us off.
The mana drain is a real drag; if the fight goes on very long, the healers will go OOM.
Perhaps the idea is to kill him slowly enough that the safe bubbles will be available for each Call of the Mist. At the rate we were going, we were getting a lot more DOTs/drains than safe bubbles, and could not have continued much longer.