From my relatively limited play experience with the big boy, and after many hours of wracking my brain trying to figure the best way to do it, I feel pretty confident in saying that Grim is the warlock that'll run the Mountain King the best. Here's my thoughts.
Problems I've found with the Mountain King: For one, he needs extra threat range. Second, he needs to do as much damage as he can right off the bat, because he just doesn't stand up to abuse. Third, he requires WAY too much support to make all that happen. I feel like Grim fixes all those problems. For one, bait the line gives him the extra threat he wants every turn. Second, Grims feat effectively ensures that the MK is going to get where he needs to go before getting counter charged. And those things mean you don't have to waste points on stuff like axers, and you don't have to choose between rush and, say, flamming fists or rage.
Anyone tried the MK with Grim? I'll be doing so tomorrow evening.