once figured out its pretty easy. first get a machine gun in a good position so it'll use most of its range. upgrade it to 3 or 4. then get that electric tower for 200$ and upgrade it to 5. as soon as you got enough money build sniper towers and later on rocket towers. just in case, you can use some ice towers so the rockets hit. and also some range boosts. thats it. you can pretty much just lean back and watch for the last 20 rounds on easy and normal.
most of the time i dont know where my character is... just out of sight. a little arrow to show whether hes left or right from the screen would be nice
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