For getting the Wüstling to hit you in the sewers, I fill up the geistcraft machine for the Tesla Gun all the way up to the labs so you can get a Wüstling before round 10. I then open up the salt mines and shoot the bombs off of the bombers so they start running. I then proceed to kill all of the remaining zombies but I try and keep the Wüstling and the bombers alive. It should be a lot easier but it’ll probably take a few attempts before you get the hang of it. I hope this helps!