Scrapping the old one (I looked around the thread, it really doesn't fit), I'm pretty interested in doing a composition for this boss if that's a good idea. We have the idea of how the boss will be, I can make something if I'm allowed.
I think [the] dreamer's theme should sound "messy" to further imply that you're battling in a disintegrating dream world, something that sounds disharmonic and harmonic at the same time, something like Giratina's battle theme, or maybe N's.