You need to improve your code to prevent the possibility of simultaneous attack from opposite sides. However you envision the creatures behaving, this is a game, so the point is it must always be at least possible to win.
You are right. It is still possible to flamethrower both sides if you are attacked simultaneously, but the buffer is too small so it's harder than I intended the final level being. I've updated with v1.1.0 which should prevent the monsters from attacking on opposite sides at the same time.