The best tactic at the start seems to be just to ignore the three skellies and run past them. No reason to waste any ammo on them.

You should really remove that car since obviously players will wonder how to get to it - I myself included before checking out this description.

I wasn't fan of the very first sewer section. There couldn't be much of any complications at all. Any random behaviour from enemies will screw you up instantly.

Otherwise, the route to the end factory and the climb back to the surface was pretty good. At the very last stage, what was the point of two headless zombies in the carage? Seemed a bit random.

Having enemies of each faction feels a bit messy thematically speaking.
The level is unnecessary long IMO (took me ~45 minutes to complete). I would consider cutting the first and last stage and focus solely on the sewer aspects since that's where much of the novelty is. I think that would make the whole design more cohesive in its entirety.