This one is kind of easy because you get an infinite amount of time in between levels to think about it. One of my friends mastered this in college and I found a good strategy from watching him. But, I suck at the new ones because all it is is trial and error and putting tons of time into finding a good way to do it. On this one, you don't have creeps coming 10 seconds after the level ends so you can think about what to do next.
The main goal is to build up the interest. only use what you need and sell what you aren't using.
