Finished the last level... It is all about timing.. And stacking gold after using your yee-haw. Usually let them mass up and let the 4000 yee-haw off, followed by the 3000 (if you get enough kills). I lost many times while waiting for the 4000 to charge up instead of using the 3000 to save my base... It's all about timing. Good luck guys, these are just tips but if you find better strategies just say it :)