The tent trick worked. He broke the crate a few turns in but I just put my party near the lowest barricade at the magister dig cave so he couldn't leave, and used archer / mage / 2 bone widows to kill voidlings as they trickled in. Thanks for the tip! It made the fight much easier, if a bit boring as I just ended 4 turns without doing much of anything while voidlings moved to me.
