First upgrade 'skillpoint earning speed' to max. Then upgrade 'money earning' to max, following with reloading speed and accuracy. After upgrading all of the stats, start growing plant. By the way, don't forget to buy x-ray gun or heavy machinr gun before growing the plant to 100%, you will need at least one of them at the final fight!
 "Nuclear Plant"){kind=icon}
 "Pirateers"){kind=icon}
 "Epic War 4"){kind=icon}