I am not in the mood to rate everyone this time, but I want to leave a comment here anyway.
I love the theremin! Every time I hear it in Mario soundtracks I think to myself, "I wish Musescore had a theremin sound". Maybe someday I'll get access to it too.
With a lot of the 'final bosses' I've heard, this feels less like 'ULTIMATE POWER' and more like a 'joker in the basement, secret fight.' The waltz at the start fits it well (alongside the voice ofc), and transitioning away from this is an interesting choice to go back to the 'standard boss 4/4' tempo.
I always love to see time signature changes, and so I love this 3/4 -> 4/4. The first beats of that 4/4 somehow fit well with the 3/4, so the transition is not jarring. I also find it interesting how you switch a lot between rhythms. That keeps it fresh every time.
Well done, as always :)