During Mission 5: Red Star Rising, you will need to kill a total of 5 enemies using the Buggy. Progress will save when you reach a checkpoint, so keep in mind that if you die, your progress from your current life will be lost. You'll come to a part where you descend down a tower, then some rocks to a buggy. At this point, you'll want to get in and focus your fire on the helicopter to take it down first, as it will be the most troublesome. Next, you'll want to get out of the buggy, and mark the enemies that are located at the top of the structure in the middle of this section. There should be a total of 4 enemies which you should run over, then change your buggy if it is damaged and trigger the next checkpoint. Then you'll come to the section with the HUGE gate blocking your progress. To the left of the gate entrance there will be some CELL soldier enemies that you can run over to finalize the achievement. If you still haven't attained the achievement, progress through the gate, and you should find 1 more solider enemy near the middle of the road on your way to the final base.
Location: Mission 5 During the fifth mission you can drive a buggy. You have to run over 5 cell soldiers. At some point during the mission you will go down some stairs and a cutscene begins to play. After the cutscene you are on a mountain. Jump down and a buggy will be right in front of you. There are 4 soldiers in the first area. They are on some sort of metal structure that looks like a ship wreck. Do not kill them with the buggy rockets! Run over all four of them and continue to the next area. Keep driving until you come to a closed gate. There are some more soldiers hiding behind containers to the right of the gate. You must run over 5 soldiers in one playthrough! You cannot farm the kills by restarting checkpoints or replaying the missio
Just to add to this I have had quite a few buggies and managed to run over 4 people. But I couldn't kill the Ceph in the buggy I just seemed to push them along. So I got in the armoured car/tank. I ran over a scorcher in this and the achievement popped. So the armour looks like it counts too.