The best way to achieve this is with the Wizard. This method will get you both Grunt Slayer and Orc Slayer. Load up Dungeon of Discord on Hard mode. This is the Level that is one room with 4 sections blocked off with Key Walls.
What you want to do is move the blocks to get two keys and take down the Left wall that contains the Grunts and the wall directly across with the Orcs. Now you move you Wizard to the bottom wall of the Orcs on the right side and spam the Ice Beam. What I did was found something heavy that could be placed on my mouse to keep the beam firing and left it AFK to get all the kills.
I used my Xbox 360. What I did was stick a coiled up ethernet cable underneath the right side of the Xbox, held my mouse down and placed it behind the Ethernet cable. This way there was enough pressure to keep the button pressed but not enough to slide it away.