A yellow wisp will be floating around the map through different walls. You will be able to hear Gersh talking when you are near. As soon as you see the wisp, start damaging it with Pack-a-Punched weapons and Gersh will start screaming. He will eventually teleport to another area of the map where you must continue shooting him until he eventually ends up in Dragon Command and the Challenge will end.
[pic of Gersh}