The timing's slightly off, but I can only notice it when it comes to "Breakfast, Deary", since I have muscle memory for how fast I need to switch tracks. Otherwise, this is incredible, and the best recreation of any Undertale fight I've ever seen. I certainly couldn't make this, particularly since you not only have to program the fight, but you have to make it match up exactly with the original, which is probably a lot harder. Have a great day man.
Edit: Okay wow I took a look at your repository and it's so far beyond me. I program in Java for my FRC team, there aren't nearly as many files involved as you have here, guess I won't try being a game dev lol.