Medal of Honor Warfighter

This occurs during mission 4. During it, you'll have a silenced "sniper" rifle. Your team mate will help you out and guide you through the mission. At one point you'll come upon a house with two guards walking back and forth. the guide will tell you to wait til they are together. Do that and get a headshot and you'll kill both. If you fail, you can either restart from checkpoint or go on as immediately after that there's another set of three enemies standing together where you can shoot 2 with one bullet.