Achievement is called Plosive Expletive
During chapter 3 you will be fighting a bunch of zombies you will head up some stair and instead of heading in the office door to flip the switch head to the second door witch is currently locked you need to flip the switch and circle around threw the train track area witch will lead you back to the same stairs but this time the door that was unlocked. When you head inside the door it will close behind you and a guy will throw a grenade into the room with you. You need to pick the grenade up and toss it back threw the hole in the wall killing the man on the other side and unlocking the achievement.