Silent but Deadly
Not sure if this way of getting it has been said before but the way I got it was on the level Delta Halo.
After you get the tank, you come up to a large temple structure which you'd need to leave the tank at that point. In the middle of the structure lots of enemies pour out. I just fired repeatedly with the tank at the opening as they all came out and I got the achievement.
Probably all just complete luck but it could probably be done again. Just thought I'd share it as the "reload last checkpoint" way of doing it doesn't seem to work.