High Priest Xalziks is struck by an explosion of freezing water.
I aggroed High Priest Xalziks, kept him above 90%, pulled him into his own whirlpools as they spawned, counted the audio triggers until I got at least 5 boops, then finished him off.
The achievement doesn't update on the 5th boop though, you only get it once you actually defeat him too.