One Shot, Two Kills
Kill 2 Enemies with one shot

The best natural place for this achievement is at the very beginning of Mission 9 Steal From Under Their Noses. The game will take you to a wooden bridge and you will hear your radio cut in saying "tangos spotted across the bridge, take them out." This is a great spot for this achievement because the 2 enemies across the bridge are almost perfectly lined up with each other. I suggest saving it here before you shoot just in case you miss, otherwise you can just reload the chapter since it is at the very beginning. For this shot it is best to be more on the right side of the bridge as they line up a little better from that angle. Also being flat on the ground will give you better accuracy and trajectory. Just wait until they are well lined up, use concentration mode (shift), and take the shot. It should be super easy, I havent missed a single shot out of 5 attempts and they dont need to be lined up perfectly. I attached some screenshot to help guide you. Best of luck!