The earliest tiem in the campaign that you'll run across a Deadeye is in a room of a building on the St Louis chapter, in this room you'll see two deadeyes, one up against a pillar, and one against a wall by a window, near an audio recording (collectible), out of this window there is a bunch of grims which can be headshot if done properly, due to a deadeye clip only having 5 bullets to it.
But to make this easier on you, save your deadeye and progress further on through the level, you'll come to a point where you'll see other survivors and it'll tell you do defend a VTOL against a chimeran attack, which the building it puts you in when you do, has another deadeye with a zipline, but don't use this zipline just yet, aim down your site with
and be patient, due to many chimeran soldiers (way more than 5), so take your time and be careful with your shots, and it'll hopefully pop.
I got 5 clear headshots with one clip, then another, but it didn't pop until my third or so clip was done, I'm not sure if this is due to the trophy being buggy, or the trophy was just being slow, but a heads up incase anyone else has that issue. Don't worry though, this area has more than enough chimera, if not, kill yourself/let yourself be killed by a longleg or something.