Easiest way to get badge and survive for ages, and i got to like lvl 75 using this method (don't believe me, I have a screen shot you can see) :
Select nightmare mode, object collision and devils off
Chose massive as the map get shotgun out straight away.
You spawn where the zombies entrance is, and just stay there shooting your shotgun for a while until you get a rocket launcher
Once you've got a rocket launcher, step back a bit and just continue to shoot where the zombies are coming from. That is there only entrance in the whole map, unless you decide to blow some fake walls down making a new entrance
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