For this achievement you must have shot a total of 84.39km. The level I did this on is Pont Du Fahs Airfield. You may check your current distance in your service record. If you would like this to be quicker, you may just shoot the man in the distance (600m+), the one using the spotlight in the tower and the officer sitting next to AA. Just make sure you save first and continue reloading. Here is my video:
If you have any questions, comments or concerns please let me know. Hope it helps!