To get this trophy, you must kill 10 snipers before they spot you. Spotting only counts if their circle turns completely red. However, to be on the safe side, only take the shot when they go back to passive mode. To identify a sniper, use the binoculars to mark them and they will get a distinct cross signifying them to be a sniper. Mission 7 seems to have issues so it is advised not to use the 2 snipers up on the ridge as a farming method.
As soon as you start Mission 5: Siwa Oasis, go to the right and get to the visible AA gun. Clear the area out and turn left towards the truck. Clear this area out and look towards the big, official looking building. Up on top is a lone sniper. Once you see him, tag him and save the game. Then take the shot and reload the save until the trophy pops up.
Credit goes to 360GameTV for the video guide.