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For those who're having trouble, just upgrade the original guns, get magnet and agility, upgrade those, upgrade all visuals you want to upgrade (you should be at a decent level at this point), and then start filling your slots (cheaper to buy a ton of lvl1 than upgrade), I used 2 magnets, 1 agility, 1 lightning, the 2 original turrets, and 6 rockets, and then you just upgrade whatever.
Once I was done with the visuals, I didn't loose a single fight, and by lvl 20, I didn't even have to move if I didn't want to.
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A nice bit of social commentary, when you realize just how long you spent grinding for upgrades so you could grind more to upgrade more... the incentive power of a carrot on a stick.