This trophy is pretty self explanatory, and pretty easy to get early on in the campaign, also.
I found the Fareye to be much easier to accumulate headshots with, due to when you hold down
, it slows down time, giving you that much more of an advantage with getting a headshot. If you missed getting this trophy during chapter 1, the level "Battleship - Orick. California" will have a whole bunch of Marksman throughout the chapter, a Fareye near the end, along with a bunch more Marksman, just remember to aim for the head.
I was able to grab this trophy before the end of the Battleship level, due to missing my chance in chapter 1.