Act 1 - "Hunted":
After killing the soldiers harassing the old man at the start of the level, you will move into a field, where Cpt. Pierce tells you to lie down to hide from the Helicopter. For this achievement, you want them to find you anyway so just stay standing, but stay towards the side of the field you came in on, so that if you have to retry the checkpoint you don't have to run all the way back to where it is. The Helicopter will hover there, and 7-8 troops will rappel down.
The troops on the left rope start slightly after the right, so focus on just the left side to have a bit more time. If you had to run over to the helicopter a bit, don't even try to get the first guy, and instead focus on the 3 that follow him. Headshots kill immediately, but are difficult. Instead, aim for their arms/body. 3-4 body hits will cause them to fall from the rope, killing them if they are at least 5-10 feet off the ground. The checkpoint is immediately before these guys, so it is very easy to retry if necessary.
Feel free to ask any questions, and add me on TT/ TA/ PSN/ XBL as Danokitty. Happy hunting!