In Mission 5: Red Star Rising, when you go down some stairs and a cutscene begins to play, you will be on a mountain when it ends. Jump down, and a buggy will be right in front of you. There are four soldiers in the first area. They are on a metal structure that resembles a ship wreck. Run over all four of them, and continue to the next area. Keep driving until you reach a closed gate. There are some more soldiers hiding behind containers to the right of the gate. Run over a total of five soldiers to get the "Roadkill" achievement.