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.
100k - 24.8.2011 | 200k - 2.12.2012 | 300k - 1.12.2013