During beginning of the level DEPOT, you will be inside of a railcar heading down a track. Just take out a good long distance weapon, and face forward in the railcar. Your teammate will mention that you should "be ready for anything". After the railcar takes a left turn, aim forward and keep an eye out for a light source. As soon as you can see a light, aim at it and shoot it to kill the enemy with the headlamp.
You can use a good 1-shot kill weapon, aim for the head, use military grade ammo or even shoot the enemies leg to slow them down. Kill them before they reach the station to unlock the achievement. Load Last Checkpoint if you fail.