This is a relatively straightforward acheivement to get - just get 50 headshot kills throughout the main single player campaign.
Headshots count whether they are via the sniper rifle or pistol, and also continue to count if you die and replay checkpoints. That said, there is very little need to grind this - there are 10 missions in total, with between 40-60 enemies per level, so you will probably unlock this well before halfway through your playthrough.
The only issue to mention is that on casual/medium you recieve aim assist (a red circle shows where you will hit) and as such getting headshots is straightforward. On expert you do not get this, and given the difficultly of wind/height/distance, it is often safer to go for body shots. That said, you can still get this on expert, mainly through closer range headshots (under 100m you can just aim for the head), and so you should still be able to unlock this on your first playthrough.