As Jill make your way to the First Floor Right Wing of the mansion when its assessable (requires Sword key you obtain after using the Arrow Head to access Mansion B1, the key is on the back of the Book Of Curses). Once on the first floor right wing. Go to the room that has the Shotgun mounted on the wall. Remove the shotgun and trigger the switch. Return to previous small room and wait for cutscene with the ceiling lowering. After the cut scene check both doors and it will trigger the 2nd cutscene where Barry saves you. The achievement unlocks after the last cutscene.
Important Note: You cannot have the broken shotgun with you, if you do it leaves the door to the shotgun room unlocked and assessable. If you have the broken shotgun, store it in a Item Box.