The boss, both the Ice Elemental and Armored Dredge, will “fixate” on all 5 players and then freeze, not moving or doing anything. The boss is still kill-able, but will take considerable time if you’re not lucky enough to have him freeze beneath a fire cauldron.
We reset by dying the 1st time and there was no change other than his life refilled. We exited and then tried to enter again. The second time we got the Ice Elemental boss and the same thing happened; all 5 of us got a diamond over our heads and the “boss is fixated on you” message. The Ice Ele just stood there and took his beating.