Near the end of chapter 4 you will come to an elevator. After you push the button to call it, enemies will swarm you and you have to defend your corner for about 30 seconds until you can enter the elevator and go up.
It's IMPORTANT that you save at least one shotgun round for the next section. Exit the elevator and shoot Ruviks brain in the middle of the room. After a short end credit interruption you can control your character again and the achievement/trophy will pop.