This guide shows one of the easiest places to get the achievement.
How to Get it Done
On mission 6, if you work your way around to the left from the starting point and stealthily kill all the dudes along the way, you should stumble across a spotter. If not, use your binoculars to find and kill a spotter. You need his flare gun which can be looted from his corpse. Then keep moving up the coast on the left where you will find a pillbox. There will be 6 or 7 dudes hanging out above the pillbox and these are your achievement homies. Climb up the pipe near the pillbox on the left, get your flare gun ready. Stand up and try to put the flare in the middle of the group. They aren’t very smart as they just hit the deck. Needless to say, that doesn’t help them, but works out very well for you.
It’s a pretty easy achievement if you know how it works and have a good spot to execute. Pun definitely intended. All you need to really be careful about is not aggroing when you get close to the target. If you do, they will scramble and won’t likely reassemble. Anyway, the video below is the money shot. Good luck and happy shelling!