This level is riddled with enemies. It is a good idea to attempt to stealthily kill as many enemies as possible, then make your appearance known so the remaining enemies can call reinforcements. You can kill them however you wish, just as long as they die. Check every area/room to ensure every enemy dies. The three main areas are the areas that are able to call reinforcements. The reinforcements are large armored enemies. Do not knock out any enemies when you come in close, doing so will void the achievement. The achievement will unlock when you kill the final enemy.