The "cure" for that Black Priest issue is... beat on in for a couple of seconds to remove the Tserrina buff as your ENC strips it with Blast, then hit your insta-cure clickie to remove the debuff on your toons. He goes down normally after that.
I have never seen the OP's issue.
My tactic for this fight is to go outside and smoke a cigarette for a while. I just have to make sure I come back inside by the time he starts actually dying and before the bats spawn. lol
