On the level REGINA, you'll come to a section where you use your railcar to push another railcar forward and out of the way.
To get the achievement you'll want a good automatic weapon such as the Kalash or Bastard Gun, and you'll want to equip Military-grade ammo, and aim behind yourself.
There will be a total of about 5 waves of enemies, with 3-5 enemies per wave. As soon as you finish pushing the other railcar to the end of the track, the achievement will unlock.
If you fail at any point by taking damage, make sure to "Load Last Checkpoint". You can also "Load Last Checkpoint" after gaining the achievement to get your military-grade ammo back!