when I did that encounter I made sure my mage had a first turn (cuz I always spend 1st turn buffing with adrenaline\flesh sacrifice for 4-5 ap...and cast something big on a last 1-2... mass CE in my case). it killed everyone and on the next turn he had 6 ap kill wave 2 and 3.

also iirc to prevent CE from enemies you can use bone armor\bloatet corpse or similar spell...

ps: now when I think about it that encounter was indeed very buggy. but at least not as buggy as the battle with paladins inside arks(where you can use phoenix totem(or invis) to end the combat and everyone will be oblivious to what you've done.