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Someone else suggested that the bug is that it triggers your thorns on yourself. I had the same thing happen and I had lots of thorns, so that might be it.

Hover over him to see his abilities. He gains regen each turn, and gains thorns and rage when you hit him. The regen isn't a big deal, but the thorns can wreck multihit strategies like shiv or cleaver. Ironically, thorn+turtle works pretty well against him; with enough defense and at least 2 thorns per turn he'll eventually die if you just don't attack him.

Oh, that's what killed me! I had a decent cleaver run going and I figured I could ignore the slimes because I had room and I had the heal gem (which is busted, incidentally). Then I suddenly died. Guess it was the 16 thorns.