Go to the railroad level on easy. Get to the location where there is a mini cave system that overlooks the bridge. Using your binoculars tag everyone visible on the bridge (Including on the top and under the main railway on the platforms). Make a manual save. Using your rifle shoot and kill everyone you tagged. Once they are all dead/you have been spotted/attacked, reload your manual save. Rinse and repeat until you get the achievement.
Each kill is approx 140m/150m and when I did it, I was able to tag around 11 people so per save/reload I was accumulating around 1540m/1650m. Given that I started this process when I was around 70k, it took me around 20 reloads. This was the best way I could think of and it took around 1/2 hours but got there eventually. :)