Super Castlevania IV music is damn hard to re-arrange.
The people who created the original tunes seem to jump all over the place with samples, tempo, effects, and melodies. This is why a lot of CV4 tunes have taken me years to finish. I only finished "The Forest" and "Secret Room" very late last year, and those were in-progress for quite some time.
Both Boss Battle tunes (castle outskirts, and inside the castle) are difficult. Someday, though... someday...