Mission 17: Loki
Progress through the mission until you reach a control laptop where you take control of a remote drone that guides rod missiles. At this point you need to make sure to kill all the enemies (marked in red) and not kill any allies (marked in blue). Use 2-3 shots to take out the first group, and ensure that no enemies survive. Next, wait for the train to pass a little bit, and when the next group of enemies appears be careful so that the blast doesnt kill any of your troops near the tracks. After this group, more enemies will appear. Start with the group in the back and make your way around.
It can be very hard to tell if you killed one of your own allies. You can tell that you hit one if they say "Thompson, make sure your shots are on target!". At this point, restart the checkpoint immediately. Also restart if you missed an enemy.
The achievement will unlock when the camera goes back to the space station. If it doesn't, you have to restart the entire level.
Note: You don't have to take down enemy helicopters for this to count.