* Build up the rewinder upgrade first. It will allow you to collect more
  coins at the end of each level to buy the other upgrades with. 

* Study where the colors are on the fixed puzzles (control parts) as the
  level starts, and try to shoot as many asyou can when you get a window
  of opportunity, while moving waves aren't in your way.

* Do prioritize the fixed puzzle/control parts over the moving waves 
  unless the waves are nearing the exit. 

* Glance at the receding bar at the bottom of the screen. It lets you 
  know when the next wave will come.