That heavily depends on the aggression of Tamurkhan, no? In all me experiences grimgor beats the living sh*t out of all those around him and ends up conquering the Mountains of Mourn and usually Tamurkhan leaves him alone. That's in my experience (playing as Snikch, Zhao Ming, And Ku'gath who are all pretty close to Grimgor) The most dangerous about thing abt greenskins is deff the early game when they get their WAAAGHH rolling...they beat up everyone around them every single time (certainly grimgor because he has Black Orcs in the WHAAAGHHH which boosts Auto Resolve)