Awesome graphics. Needs some work though.
It's a good try but you're probably getting a lot of low scores because of the difficulty.
I thought the different enemies were pretty inspired. They're similar yet different enough to make you change your strategy almost every level.
Here's the problem: It's too damn hard. I stopped playing after the green bubbles. It's not worth my time and frustration to slowly whittle down their health. I don't mean that in a mean way. I'm speaking as part of a large demographic of casual gamers who want to have fun too. If you don't include variable difficulty you're killing your appeal.
Here's what I'd do if I were you. Take a page from Death vs. Monstars' upgrade system. If I can't beat a level. Let me grind a little bit until I can buy power-ups that let me beat future levels.