Great first impression with the opening cutscene and how the game looks in general. I like the toon shader and the music and sfx.
As for the gameplay, well... I suck at rythm games so I did not do too well in that phase. I wasn't sure when you are supposed to press the button - when the object is above the flute or behind the key button? I managed to get some points there, but I'm not sure how :D But since I did so badly I saw the boss monster transformation - super cool!
During boss fight the boss glitched and got stuck hanging in the air. It did nothing and I killed it in a couple of hits. Is there any way to play the boss fight without it breaking? That seems like it could be fun! Also, attacking during boss fight shows collision shapes for a bit - not sure if intended, or it was there for debugging and was supposed to be removed for release.
Great job!