I love the implementation of the rhythm part, it was very neat as the music plays when I hit the right note so I know Ive hit it right. Lovely music tracks and the notes are placed right on the beat as well. I think the tolerance for hitting note is a bit too small, I miss a lot of notes but can't really learn from it that next time I should be earlier or later than the threshold I use myself.
As for game design, the first two NPCs are pretty weak so players can fight them while still making a lot of mistakes. But you make them disappear after I defeat them, which I don't think you should. Here's why: You gradually ramp up the difficulty to prepare the player for the final boss, but still allow room for error. Then the player goes to the final boss level with very fast music, difficult note placement and terrible skills. If he loses, that very difficult track is the only thing he can use to practice timing skill. I would love to be able to revisit the defeated NPCs and practice more with slower tracks to perfect my timing before going to boss level. That's my only nitpicking suggestion. The game is very potential for future development, but I think you should find a way to make it stand out from all of the games of this genre, or put in a lot more work to polish it. Good luck!