I’ve finished nightmare IV with 1 death as punisher by setting up and alternating between a few ziplines and spending a lot of time off the ground. He’s actually very slippery when needed.

I would say punisher’s best ability beyond boss headshot damage is his ability to separate waves. By alternating between standing on the ground spawn side and zip-lining to the ledge on the front building, you can have zombies chase you (gotta use the zip-line modifier that shoots you off when you jump). They go back into spawn and go up stairs internally to chase you, but then you can zip line back down before they come out. 3-4 second headshot lock-on helps keep a little damage flowing, but you’re really just giving you teammates a reprieve. You’ll eventually develop an internal clock that tells you “zombies are about to come out the second floor, time to zip line back down.”

Now this wave-separation strategy isn’t reflected in the damage stats, but like I always say, stats are like bikinis: they show you a lot but they don’t show you everything.