Grah-Hoth isn't that bad. When he loses a certain amount of HP he spawns adds. Kill them, attack Grah-Hoth, kill adds, etc. Once he spawns the quickghasts hit him with everything you have until he says he's going to stop summoning help. Boom, easy.
With Hawthorne you had to motor every step of the way because of the endlessly respawning Royal Guards, Mages, etc. And the respawning wasn't even a matter of "you killed mob A, so A will respawn in x turns." It was "here's mob A, mob B's coming in x turns whether you like it or not, and so is mob C, and D, etc." And then you had to attack golems which blasted the entire room with those novas. If slow doesn't work, it gets nasty.