AFAIK the kill ability is random, it just fucks you at the start if he has it as only you have a minion out (it also hit my Boom, Onyxia, etc every time instead of a 1/1 fwiw but there where times it killed one of his minions instead of mine if he had some down).
RE the Priest deck with spells, you still need to mulligan for basically double mind blast and light bomb and he has to mind games within like the first 2 turns or he will turn into rag and kill you with his hero power, good idea but even more draw dependent feels like. I ended up just out valuing him with some Sylvanas/KT luck but I thought the fight was total shit, which is dumb because it was really fun on normal.