When following below strategy, keep in mind that you won't need to upgrade your shotgun, as long as you make sure you keep buying gunmen as you progress. The chainsaw makes things really easy while saving up for the mini. I upgraded the damage on the chainsaw about half way, and only bought a couple levels of reload, and then left it alone.
Basic strategy for beating Normal Campaign follows. Buy at least one gunman every day while saving up for the shotgun. Continue this pattern while saving for 2 quakers, then a laser. Next, buy a chainsaw. Continue buying gunmen and a few repairmen until you can afford the mini-gun. Sell one of your quakers, buy another laser. Upgrade defenses and mini-gun as $ permits. By the end I had 80 gunmen, 30 repairmen, fully upgraded mini, 2nd best wall, 2 fully upgraded lasers, 1 fully upgraded quaker. Good luck!
You don't even need to do anything. Just buy a fan, fully upgraded, and put it in the middle, and 2 fully upgraded flamethrower guys on the top and bottom. Buy 10 repairmen, and spend the rest of your money on gunmen. They probably won't even get to your wall, and if they do they'll get one or two hits at most, which will be instantly repaired. Sit back and enjoy the crispy carnage. :)
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