I think this difficulty is done in order to check YOUR wits and not your PC's abilities. Obviously the direct route is inappropriate in such cases. The challenge is to manage to lure those minibosses into various kinds of traps and hit them while they cannot hit you. Try using a bow for this task. You can create barriers with barrels or boulders in confined rooms so they cannot reach you, you can freeze them, you can throw steelscorpions and NOT shoot them so they just try to destroy those almost indestructible steelscorpions (steelscorpions are SO very powerful, they are actually essential. Throw 10-15 of them in a cave, let them wander a bit, go make coffee or eat something and then voila! the cave is free of almost every monster, however powerful, with the XP in your pocket!)
