My tip for beating survivor mode is this: Because the game starts off super easy, and gets tougher as you go on, you should try to avoid building towers that you later sell and rebuild with something else - that's an inefficient use of cash.
Maximise your cash, so that later on you can build the high-end towers you'll need to finish the game.
I've had a few perfect games in survivor mode - to do so I usually build 4 teleporters, to make sure I can teleport all the aircraft that come along in the later waves, and about 20 fully-upgraded SAM towers in good locations.
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