Hehe Wizmirror, I like that!
The game was made in PICO-8. It's a great engine to get started in game development, and if you're interested I'm working on a tutorial series to help people learn PICO-8. The music was created in the chiptune editor that comes with PICO-8, which I found to be pretty accessible! You can even download Just One Boss and export the music to a .wav file really easily.
I love boss fights and I have a dream of making a full boss rush at some point in my life, but it probably won't be with this set of mechanics (though I do think these mechanics have a lot of interesting design space to offer).
Thanks for playing :)