For boosting of this achievement, I would recommend mission 9, where you have to kill Moller.
Right at the start of the mission wait until the car with Moller on it gets 900 meters away. At this distance, Moller and the escorting car each with 2 soldiers on it will be moving right away from you.
Save the game before killing them (soldiers first, then Moller), then reload the game. You normally can kill 4-6 each time, which is 3600m in total. It will take around 20-30 minutes to accumulate 100000m.