Calidron, make sure that you are completing all of the challenges. Its sometimes necessary to replay levels to get experience to level up your base and unlock new buildings and weapons. If you haven't already, make sure you do the recycling plant challenge as it starts you off with an extra $100 to begin with. This makes it MUCH easier. Secondly, make sure you do the challenge that allows your gates to auto heal. This can be the difference between getting perfect and not. If you can't see these challenges, go back and redo levels to get to the next rank. Make sure you go into a level knowing what you need to build to complete challenges. Once you get to the last few waves, build what you need like crazy and keep scrapping them to get your money back (recycling plants help with this, I generally build at least 3). I hope this helps.
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