FOR PC PLAYERS:
1. Start a new game - select single player and Scourge of Armagon
2. Now hit the ~ key and enter this "map hip2m5"
3. Enter "god" and "noclip"
4. You are looking for an exist arch. It is near the rear of the level, on the edge of the level, on about the same plane as your starting location. It will have a gate over that looks like teeth. You might be able to noclip through it, but I turned noclip off in front. Hit the red button and pulled the level right next to it and then it opened and I got the achievement.