it seems he is on a 12min respawn timer, engage him when repop, else he wanders around for some minutes to get back into the water. Under water there are nearly no adds but strangely, no spell lands, but the mercs can kill him melee. But if you get over water you get wasp as adds. Easiest: stay under water, attack and then go back and let the mercs and pets do there work while a healer merc heals the tank merc... they stay under water relatively safe....