I got it at the rooftop level, beginning of act 3. I only picked off the guys at the stationary guns, aiming at the lip of the helmet. You can get around 6 kills from these gunners, but after waiting a minute or two, you will get a "objective complete!" text, and then there will be more soldiers at the right side stationary gun. Then I just finished a few more headshots and got the achievement. I was playing on easy.
GSL 13 starting score: 330,305g | 100k - 24.8.2011 | 200k - 2.12.2012 | 300k - 1.12.2013
My top 10 shooter completions: Gears of War 3, Max Payne 3, F.E.A.R., F.E.A.R. 2, F.E.A.R. Files, Rainbow Six Vegas, Warhammer 40k Space Marine, Gears of War 2, Legendary, Left 4 Dead 2 (Realism Expert legit Dead Center & Hard Rain) | XBA shooter leaderboard: 132,764 GS (11th)