There are various shortcuts in this level, most importantly there is one near the beginning that allows you to skip most of the enemies in the first section by taking a series of pipes to the ground floor. You can also use a trench near the side of the railroad, and some pipes in the last section of the map in order to completely avoid enemies.
I recommend watching the video to see the best path through the level, which makes it super easy.
Note: This achievement requires you to not KILL anyone, but "knock out" is fine. Also, you might have to do this achievement in one go from the very beginning to the very end of the level as I had trouble getting it to unlock when I used the "Load Last Checkpoint" or whenever I died.