In my opinion, the most annoying and elusive trophy on this game. What you really need to do is kill two hostiles at the same time with one bullet (aka: double kill), and you have to do this FOUR TIMES and finish the mission successfully, the trophy will pop at the end of the mission. Only bullets from the sniper rifle counts, so double kills from any other weapon (i.e. the rocket launcher) DOES NOT count. You know you made a double kill because you'll get 200 XP instead of the regular 100 XP.
Needless to say, play the mission on the easiest difficulty. You need to try SEVERAL times and study each enemy position. You'll have more than 4 opportunities to score a double kill, most of those will be a fraction of a second when a enemy runs behind another and you have to shot at the exact millisecond to score the double kill. A couple other opportunities will require that you "soften" one of the two hostiles first shooting him a bit with your regular gun (without killing him), otherwise he will survive the sniper rifle bullet; you'll start to notice these cases as you practice the mission. Remember: if a hostile gets behind another one for just a millisecond (even if it is his head that aligns with the foot of the other), that's an opportunity you have to take your double kill. Also, keep in mind this trophy is a bit luck-based, as sometimes the enemies will not align as they normally should. Just keep trying, it took me a couple of hours of constant trying to get the trophy.