This is the fastest way to get to the hells retriever, on the 2nd round.
- Feed 5 zombies from the first room to the dog upstairs, don't shot the last zombie
- Go to the showers and turn on the washer to release 20 more zombies
- Get the 2x that is in the cell locked by the volt meter and knife only 8 zombies, don't shot them even if u get a max ammo (the more you shot the higher your chances of getting a nuke)
- Go to the infirmary through the cafe and knife 6 zombies to finish feeding the dog
- Open the door worth 1k points on the top of the infirmary that leads to electric cherry and the gondola
- Go to the docks via the gondola and feed the last 6 zombies to that dog (you might get a nuke so finish feeding the dog on round 2)
- Go back up the gondola, make your first right and drop down and open the other 2x that is locked in the cell
- Now you can shoot 8 times and knife until you reach 1,500 points to open the door by the warden's office to the catacombs.
- Enjoy your hells retriever on round 2
Here is the video which might help you out better. This is the fastest and can't be done any quicker.