Medieval Moves: Deadmund's Quest

Critters are the animals wandering around during each chapter that are you're not required to kill. Some of them are on the ground, others are flying through the world. Just be mindful of these and try to free a few here and there during your playthroughs and you should be able to get this. If not, you can farm this by finding an area with a few critters and allowing yourself to be killed after freeing the critters as dying does not reset your kill count.