Wow, I loved everything about this, the combat is good, and hard! Took a while to get used to it but after that it felt very nice, I think difficulty wise it's pretty well balanced, you get better as you keep playing. Graphics and sounds are great as well. Only thing I would improve is the "icons" of the power ups to be clearer or bigger so you can easily tell what they do, and once you play a couple times you recognize them easily. But it's a minor detail, great job!
Edit: maybe just assigning different colors for different effects would be enough, not sure how many power-ups are!