This can be done on any level, but is most easily done on the level Dark Star during the section where your railcar stalls on the tracks.
Use a combination of grenades, high powered pistol and the flamethrower to kill the enemies as they approach the railcar. The flamethrower (which is automatically given to you) is good at killing enemies that jump onto the window frames of the railcar. Just keep killing enemies until you get the achievement.
If you take any damage just "Load Last Checkpoint" in order to try again. You should be able to get over 20 enemies without taking damage quite easily.
*Posting for Maka91, because he is only playing the digital version.