This secret achievement is an easy one, so you may get it accidentally ! All you have to do is to shoot any enemy that holds a door as shield through the hole with any weapon. A nice opportunity is during the second mission Episode 1: Billy The Kid - A Fistful Of Hot Lead when you are near the clock tower an enemy will approach with the door as shield, shoot him before he come too close or he will pull aside the door and you won't be able to do it. Check this video for a visual demonstration.