Mage and Rogue can both clear the "orcs". It helps to either hold down Shift (to run) to get them, and/or try and corner one or two of them in an enclosed area of the farm, as they'll respawn pretty close to where they're killed.
That being said, I usually have to give it 3 or so tries before I can clear that stage myself, so I'll post an update with a lower quota.
Thanks for playing, and for the feedback! Much appreciated.