Easiest way to win: standard turrets. Buy them at first, and by like day 3, you'll have enough to get another elevator so you don't have to jump levels. Fill it up with stock turrets and just let them do most the work. You can upgrade a few of them, but eventually it gets to the point where you can splurge on more elevators and laser turrets by just using those simple cheap guns
Don't use the nuke!! You can blow up all the barrels you want at point blank, but nuke and splash damage finally becomes a factor. Game over man. Game over!
The best strategy that I've found for the final campaign is to get three plasmas and level them up first. When you can afford it, get a drone. After that, damage buffer. Upgrade only the attack of the plasmas, the ability to fire/health/# (that order) for the drones and the damage % increase for the buffer. It makes the final campaign laughable :D see?
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