Got to wave 17 and quit from boredom. Harder please! Much, much harder! (reppin QGMLWB by the way http://mkweb.bcgsc.ca/carpalx/?full_optimization )
Did you try starting it in expert mode? If you did level 17 there, my respects to you :)
The only trouble I have is if I have to take a break and they advance a bit. My gun seems to lock on things I don't expect it to (missiles). Otherwise, they can barely make it on to the screen.