insaneintherainmusic is probably the best arranger on YouTube (partly because his work is jazz-focused, a rarity in the cesspool of rock/metal remixes; partly because he is SO GOOD at it), and the fact that he does videogame music (especially Pokémon music) just makes it that much better.
The odd thing is- I remember going through Dark Rust Fortress and seeing the credits, but I have 0 memory of beating the boss. I do like the insane mess that is this music though. At one part it sounds like your game system is breaking down as it does a slow 'skip' at one point. I used to hear those sort of skips on my CD player and Playstation as my discs were scratched and it still makes me nervous. XD
Found this in my sub box on YouTube and I love it, the original was one of my favorite tracks when I played through the game.
If you like music that is beautiful and epic at the same time, I recommend this masterpiece. I've been listening to it for a few months pretty much every day. It's amazing.
Heard this song a while ago and thought it would sound better as a nightcore... and wasn't disappointed. xD