This trophy is a bit of a pain to be honest. The main reason being that even on normal difficulty, most enemies wear helmets. It also has a habit of not counting headshots sometimes.
The best places to do this are in the second or third enemy encounters in Cozumel when you have to get through the dig site. In the first encounter, if you kill all 3 and don’t get it, it will auto save and you can’t reset. So make sure before each time you try to switch to an alternative save slot so you can load back to just before. I would recommend trying at the second set of three guys instead though. That way you won’t have this problem as there’s a 4th guy on a ledge. If you don’t kill him you can just keep resetting to the last checkpoint.
Another tip would be to line up two of the guys in your sights so you can do 2 headshots quickly and take a beat to get the last one. 3 seconds isn’t long. You can also turn down camera shaking and vibrating in accessibility options if you want.
It may be worth noting this can’t be done on deadly obsession as it punts you back to the start of the whole section if you reset or die here.