You can get this on Mission 10 The Wolf's Den. Once you pull up the wood and drop down into the tunnel the achievement starts. Do not use anything but the 1911, no melees, no smoke grenades, nothing notta, dont even pick up other weapons! Try to use no more than 2 rounds per enemy. Line up as many headshots as possible because if you run out of ammo you cant melee them so conserve ammo.