I was stuck on level 16 for a while too but then a couple of hours later I came back, tried again, and beat it.
From there on the game was a little too easy especially level 30.
More levels would have been cooler and if you could choose that dificulty level that would be nice. (Wow is it just me or is starting to sound like a review?)
Oh and by the way, to those who can't beat level 6 or level 7, you have some work to do.