I've had a lot of success with laser tower spam - even on some of the later levels. I will admit that many required a mixture of laser/shock treatment, though.

This is a cool way to challenge yourself and keep the game fresh, though! Maybe this could be an idea for geoDefense 2 - a challenge mode where you're limited to 1, 2, or 3 user-selected towers, or maybe even randomly selected towers for added challenge.