Start playing in The Inferno level and once in the blue key room, locate the ledge on the side with the missile. Approach the wall slowly go down the ledge, For best results, allow the car go down by itself. You will fall through the ground, and end up inside the wall. You may move around and shoot people with all the weapons with the added protection of the wall. With skill you can go to every room. Note: You can be hit with a shockwave or disruptor.