I started the game with the bruiser imp and manage to get to the goblin slayer "boss"? Obviously can't defeat it so i'm not too sure what to do? I've venture everywhere else possible and it dosen't seem i can progress further.
Best bet is to unlock the short cut, hire the Thief and Iconoclast, and use their party-wide dodge and regen skills for the first few rounds. Try to kill his allies early on, Thief has a poison skill to help ignore their armor.