First, get huge, under the spawn area in the Batcave. Second, Grab a colossal assault rifle. (helps if everyone is dead) Third, get to where it's kinda like a corner. (the bottom of the spawn platform) Fourth, aim what I think is about 75 degrees up and get your mouse at the end of the barrel that's closest to the body of the gun, and fire. You'll notice that the overgrown bullets are going backward.