An easy way to earn cash (At the expense of your day count) is just using a launcher that needs no effort to use at full power, and keep upgrading it with the money you earn.
A couple of tips: First off, use everything you got when you get into the later game. Use the rage bullets to get zombie trolls to attack everything in that group of zombie. Two, take everything the game gives you. You need it. Three, when it comes to the later game, dont think about profit maximization. You have a rocket launcher, use it. Youll get profit from your team. I hope this helps
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