Every single level I just send all the waves at once to get a ton of money and use it to build, or start to build an area burst tower upgraded all the way, a ghost area burst upgraded all the way, a range boost and a fire rate boost, preferably touching both tower but separate ones are all right. If I do this, then I don't need to make any other towers and I can beat any level on any difficulty. I feel like this is a significant flaw in the game because it made it very boring and easy once I figured this out. Perhaps a solution would be to limit the number of waves you could send at once?
 "Bubble Tanks Tower Defense 2"){kind=icon}
 "Take Something Literally"){kind=icon}