In the "The Final Test" mission, from the start sneak through the area on the left side to see a pipe at the end of the building. Wait until the two mechanics pass by the pipe or they will spot you. Climb the pipe as soon as they walk into the building. Enter the room on the upper floor through the window. Place your pistol on the floor by the door. You will get the "Security Defeated" achievement and will distract the guard. The guard will then leave with the weapon and the target will be alone in the room with you. Take him out and hide the body in the bathroom box. Then, go back the way you came, and throw a coin to distract the guard underneath the pipe to get the "Silent Assassin" achievement.