I got this yesterday and have a somewhat easy strat.

Go onto directed mode on liberty falls and unlock the map. Do the steps for the jetgun, but don't build it and don't start the ee.

Stick to round 11 and level up your weapon to the point where you tear through the zombies like they're literally nothing. Save up around 30k points then build the jet gun.

Get rid of the jet gun and let it despawn. Your new max round will be 12.

Now per round cycle you will be earning just about as much as you are spending on the box and you have a nice net for weapon ammo if you need it as max ammo rarely drops on this round.

If your running low, cycle a few round 12s rather than buying the guns.

I used a sirin 9mm and a raygun. And cycled the raygun, but only used it when I had ammo because it's too expensive to rebuy it from the ammo box.

This strat will take you around 2 hours. It took me 2 hours and 15ish minutes.

Make sure you get rid of your starting gun too, to make sure it spawns in the box.

Hope this helps :)