After reaching the Command Bunker in Chapter 2, head down the path toward your next objective. Part of the way down this path, there is a mounted heavy bolter. Press B to jump on it and start mowing down the Ork, careful not to kill them all just yet. Reload checkpoint until you get the achievement.
After getting the achievement, the choice is yours to finish off the Ork here and move on ... or you can reload the checkpoint and work on
since you had just received the skill after you met up with the lieutenant at the bunker.