Since he can break even adamantine beartraps, the best way to do it is with banana peels. You get plenty of them for free, and it takes a lot of bananas to fulfill the requirement to challenge him in the first place. You just have to be careful that he doesn't kill himself via self-decapitation, but usually you can take off all 4 limbs without too much trouble.
Sometimes the sneaky bastards can be hard to find, especially when there's some other type of spellcaster who can make them invisible. Lord knows I've missed them when running tests in WM, with everything visible.
So it's not just me? I didn't get those as yellow chests either. Thought there were no new posts, logged out, then saw the "today" and logged back in XD