Act I: Mission 2: From Out of Nowhere is the easiest place to attain this achievement. After succefully saving the POI in the hotel assault, you will need to escape the scene via mountain paths. Walking up to your left there are four (4) guards you must take out to pass, three (3) huddled and talking around a campfire on the low path, and one (1) on the catwalks above you, both are accesable from the path that leads to your next mission marker. Wait for the guard on the catwalk to walk behind cover and out of sight from the group (he will stop, stealth kill) go to the entrance of the catwalk and lay prone, dope your scope on the soldier frthest right, he has a single grenade located on his left hip. Simply wait for him to quit swaying and cross his arms (not sure if he crosses arms or puts them on his back), this indicates he is standing still. Take aim and fire, it will appear after the explosion cloud passes