the game froze at the end (and I did beat it on medium). the idea of the boss battle added a neat element, and I found your choice of boss amusing. You did a good job on judging the difficulty for medium as I did die some at first, but as the evolutions came along, I was able to tackle things I wasn't before, while still being challenged.
I would have made it so the power ups were more apparent, as I didn't realize that is what they were until my third evolution. Your art style kind of clashed with the natural feel of the game, but I did like what you did with it by the end. One note, I was able to finish the game without using my invisibility, and I think that should be an achievement.