This made made me giggle at the start screen! I ran into some problems with the firefox web version: As others have said, the mouse pointer doesn't register until you fullscreen and the game froze on me a couple of times, so I had to refresh (probably because I have an old laptop).
I don't feel like the game is too easy for what is trying to do. If it was hard, you would have to repeat some waves and the humour would lose it's impact.
It's a pretty fun experience! Great job!