In the mission Opera House, you will have to move up to a sniping location from where you start at. You have to prevent every guard from seeing you. If you see a guard, make sure no other guards are looking and shoot him in the head with a silenced pistol to avoid noise. Move from cover to cover and stay crouched. There are several cameras that you have to disable to avoid getting caught. When you see a camera, look for the yellowish box that you can use to disable the camera by running up to the box and pressing X.
Make sure you keep looking at your map by pressing the Back button to find out where your enemies are. If you alert a guard, you can just restart from the last checkpoint and still unlock the achievement.