Some tips - this achievement somehow took longer than expected.
- the hardest are the 4 carts with 2 men each - kill both in every cart. And yes, they must be shot (letting them crush doesn't count).
- if you think that you have missed one of the carts (usually the driver), reload checkpoint - should put you right before the carts (part 1 and 2 respectfully).
- if stuck with a silenced smg - fire about 5 cm in front of the target! Bullets travel slower, so adjust for speed and distance (hence the 5 cm).
- achievement pops just as you shoot the last heavy on board. If you followed my tips and no achievement, finish the stage, reload chapter. Use military rounds for carts - so that you can kill first 2 carts with 1 clip.
Good luck.