Damnation

There's no way you should finish your first Campaign Mode without this trophy, headshots should be your staple diet and certainly should come naturally during your campaign progression.

Should you wish to farm it and ensure it's one less thing to have on your plate then the very first set of enemies in Arrowtree are ideal for this. Before you start, use the cheat code to use a Custom Loadout and ensure you equip the Dark Seraph Rifle. Progress to the point you encounter your first enemies, a team of 5 PSI soldiers patrolling an entrance to a tower. Armed with your Sniper Rifle, carefully headshot all 5 enemies, note that 3 are at ground level whilst the other two are on platforms higher up. As soon as you down the last guy, pause and choose to Load Last Checkpoint and repeat what you just did. Providing you don't miss (and therefore blow your cover), you should also unlock Where'd You Come From? at the same time but if you do miss, then just keep reloading until it also unlocks.