Mercenaries: To the death is bugged

Playing against Neeru Fireblade, my Grommash has to the death, and I also have xyrella. This means I can reduce Neerus damage to 0, and since his portal doesn’t have damage either, Grommash should be able to kill Neeru on turn 1, although it takes a while. However, for apparently no reason, he stopped when neeru reached 39 health and the portal 380. I assume it’s to stop bugs that lead to some infinite effect from continuing, but when you realize that you have found a way to instakill a boss, and you’re stopped for no reason, it is very unsatisfying.