For this Challenge mode to be completed, you must kill 39 of Fisk's men, not including Bullseye, without reloading or picking up another weapon. After fighting Bullseye and after he jumps down to the next level, point your weapon upwards towards on a hole in the ceiling and shoot at the boxes. The boxes will fall down and a hidden guard will appear. He will not make himself known unless this is done. If you made it to the end of the Challenge and notice that only one guard is missing, this is where he will be.