First,get a cheat system and enable cheats that you like best.(some cheats you will need)
Start up Nazi zombies at Der Riese.Then,get a Arisaka and get some headshots for a few rounds.
Now get a good gun,(if you open the left door and garage door you can access a mystery box from there)
Then enable super jump and jump the courtyard fence.This can provide a point to where you can skip opening some doors.From there you can access the right side and activate the electricity so you can access the left side.Now get up to the left side and go through the garage,take a left,and open the door.You can access a teleporter from there.Use a speed walk cheat and link it.(remember to pause and kill zombies while doing these!)Now visit where you activated the power,and open the door next to it.You can link another teleporter from here.Go to the bridge and keep heading that way to reach anoter teletorter.Link it.Now go to the courtyard to open the Pack-a-Punch system.Now jump to he left corner to go through the machine.Head left to see a tiny wall jump on it.Now you are immune to zombies and dogs.
(be careful,soon zombies will be able to jump at you,so be ready.)To get back to the courtyard,you keep jumping on the right side of the system until you come out.Let the zombies group up at the mainframe until you can throw a grenade and get a lot of kills.(leave 1 zombie alive so the next round won't start until you kill him.in this time you can get ready)Upgrade your guns until from the points you earn.Have fun killing zombies!(I do this and can survive as many rounds as I like)