When the Goblin Mech first enters on the throne room, he quite obviously spins around and shows that he has a coal furnace on his back. That makes me think this is a centerpiece in how to defeat him.
I haven't had a problem brute-forcing my way through the level by just piling damage on him, but I haven't tried on any difficulty higher than medium yet. I imagine I'll need to take advantage of his weakness on hard and insane if I plan on succeeding.
What is this boss's weakness?