Resistance 3

This trophy will more than likely come by just playing the game. Enemy snipers will have laser sights on their weapons, which makes them easy to notice. Whenever you see a laser-sight, switch to your Deadeye and kill them. You can get about 11 of these kills in Mt. Pleasant, and at least 15 more in Graterford and New York, and the kills all carry over, so you don't need to worry about missing this.

If you wish to farm this trophy, you should do it at the checkpoint: Mt. Pleasant, PA: Forest. Here you can kill the first 8 snipers before the first cave entrance, then let the dropship kill you. Your kills will remain, and all of the snipers will respawn, allowing you to go kill them again. After 20 snipers have been successfully sniped, this trophy will unlock. You can track your progress through the Extras menu, Rewards section.