Easily done right at the beginning of the game. On Siege of Tobruk you'll be told to kill the spotters, two in front of you and a third who appears on a ridgeline after killing the 1st two. Save as soon as the 3rd spotter appears and take aim right for his daddy bags! If you shoot from the corner near the snipers nest where you shot the 1st two, your target should be around 130 metres, easily the required distance for the achievement. Reload if you miss.