Monsters are fighting evil because there some things simply are unbearable. Being monster does not necessary mean being evil - many of them have different interests. Some of them like to roll over, others prefer to explode, yet third kind shrink and disappear without a trace. If they put their abilities together the battle Monsters vs Evil becomes much more evenly matched than one could initially think.
Instructions :
Use the mouse to activate the various creatures in Monsters vs Evil. Send them rolling, make them explode or disappear, just make sure that the dark evil is pushed off screen or is touched by one of the monsters. That ought to finish it off in an instant.