When you reach the place where the elevator to the second basement is located, save the game. To do this trick you do not need the lieutenant's uniform. You need Anesthetic. Look at your map and make sure no one is looking towards the elevator and that nobody is in the room except the guard sitting on the chair. Move behind him, knock him out with the Anesthetic (use the whole bottle). Then, take the elevator down. When the guards find him, they will only look around and get back to their positions. This is a safer way to achieve the Silent Assassin rank, as you only fire one bullet (to take out the target).