This is obtained by getting the shotgun without the broken shotgun. At the start of the game after obtaining the Sword key, head through the double doors to the right of the main hall. Head through the door to the right of the dresser that can be pushed. Head through the next hall way through the door at the end. Now in this next Hallway there are 5 Doors. The first is the one you came in from, the 2nd heads outside, the 3rd heads to a bathroom, the 4th heads towards the shotgun and the 5th exits to the next hall. Head through door number 4. This will put you into a small square room with another door. Go through that and you will see a shotgun on a wall. Take it which will activate the trap. Grab everything in the room as it will become inaccessible after you leave.
Once you leave the Ceiling will start to lower. Run to both doors and try to open them, they both will not open and it will enter a cut scene where Barry will rescue you and the achievement will pop.