The trick is Heavy damage towers for land, and air towers for air. Splash is non effective post lvl 50. place as many heavy damage towers as the map will allow, then fill in every other available space with air towers. Then, as the money comes in, upgrade everything to lvl 9.
And, start very slow and minimal so you can get that interest rolling.
Game finally ended at lvl 389. had 23,000 points and 77 million in the bank.