I survived two waves but then nothing happened for a while. Wave 2/3 in the upper corner and no enemies anywhere in sight. I ran around the fight area and an enemy spotted me from further away. It was an invisible Werewolf, couldn't lock on to it or do any damage. Needless to say i had to restart the challenge.
EDIT: I restarted, passed wave 2/3 but now i'm stuck at wave 3/3. Enemies slayed, standing in the pond all alone.