Velvet Assassin

You will enter a room where you are required to pick up a key on the desk. As you enter on your right hand side on the wall you will see a painting, walk up to it and activate it to reveal a switch. Pull the switch and a doorway will open behind the desk with the key. If you are wearing the SS uniform change out of it using the wardrobe in the right corner and proceed. Climb down the ladder and follow the hallway until you come to a room, kill the german inside and the achievement will unlock.