This trophy is cumulative across as many campaign playthroughs as you need. If you start on Casual, you will probably unlock this before you complete the campaign. If you start on Expert, it is much more difficult to get headshots since you don't get the aim assist "red dot" on your scope.
Pistol kills also count towards this. Just keep plugging away and try for headshots when you can. Trophy will pop after the 50th face you destroy.