After some troubleshooting for an unrelated issue, I think I have learned a bit more about this bug. I was finally able to draw its aggro once by getting dangerously close to the acid pool. He attacked me with the poison spitball once, causing me to be out of range and he dropped aggro. On subsequent attempts to aggro him, I swam in the acid pool. This did gain aggro (and hurt very badly), but only while I was in the pool. As soon as I returned to the battle stage, he dropped aggro and healed again. I suspect the reason for this bug has something to do with his leash radius, or whatever it’s called. The proximity required to hold his aggro seems to be inside the acid pool, resulting in a stalemate.