I played mob of the dead recently and i did everything i could do solo, got the golden spork and hell's redeemer. I too thought there was a problem with the copy of the map but I tried again, I spent round 25, 26 and 27 on the brigde only killing zombies and the warden with the hell's retriver and after that it worked. So in conclusion, stay on the bridge for a couple of rounds and ONLY use Hell's Retriver.
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Atomix
@Atomix