So for context, I used to play guitar and I've studied music a little, but I'm by no means a musician.
I also play rhythm games a bit, and I'd say I'm neither great-nor-bad.
Your game is soooo bruuuutally haaaard.
It took me about 15 minutes to play THE LICK even once, and admittedly some of that was due to frame skipping, but still.
I think part of the difficulty there was due to the fact that I couldn't rest my hand, as I had to play five notes. This required me to shift my hand up one key to hit 5, then back down to reset for 3-12, which is quite tricky as is.
Add in movement and it's very, very hard.
I was able to defeat the first demon, but I feel like your mechanics are fighting each other a bit. The mouse movement doesn't really add anything to the game, perhaps the game could function more like a movement-less battle game?
I really, really like the idea of using music to fight and I understand that the one lick, one measure thing as to keep it on theme, but I think only being able to have the one lick affect gameplay gets in the way of being expressive with the music.
That said, having to actually pull of THE LICK does have a certain appeal.
What if instead of having to move around, each note of THE LICK did something? One note blocked attacks, one note attacked, etc. But you could only *finish enemies off* using THE LICK, otherwise they get some health back and get back up?
That way, you have the satisfaction of learning to play a piece of 'music', as well as some of the free expression of just hitting notes and grooving out on devils.
(as for the technical issues, I think there might be some pre-existing unity plug-ins which help with syncing game-play to rhythms and stuff, you might be able to find something there that helps with future rhythm games.)