other strategy: only upgrade when necessary, spend all the bonus points on investment till 50% after spend the rest on antimatter shield and damage reducer/heatseeker.
If you do not spend money, just when nescessary - more than one shot to kill - you can upgrade the damage and bullet spped to max at level 25. after that it is relative easy...
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 "Shield Defense"){kind=icon}
 "Bloons Tower Defense 4"){kind=icon}
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