Don't bother grinding all those 'build 100 XXX towers', because on your way for the 'Master Strategist' achievement ('Earn 100 Gold Medals on any missions'), you'll get it.
Here's a YouTube playlist made by JogandoEAndando with videos on how to beat EVERY mission on EVERY mode on NORMAL difficulty. I've used it from A to Z to beat the game (50hrs) and each guide is perfect. Very nice work from him, congrats!
https://www.youtube.com/playlist?list=PLyx3ruPOCg3_g0ZCN_9in...