Gaping Spider Recluse is the only pet that did any sort of damage to this monster.
We found the Gorax up on the far northern plateau on western edge area. He is solo, roving with no lair. Despite large group and health and mind buffs and pets with sweesonberry roll buffs and doctor buffs etc etc. this guy kicked our behinds and killed almost everyone and pets. No hope at all I think. He must be at least 10 stories high and does area damage. Nasty.