Yeah, Marluxia on a level 1 run pretty much requires perfection. I wonder how many people will get stuck on Roxas too, especially considering he's probably the most difficult mandatory boss in Final Mix. And yeah, aside from the aforementioned Collision Magnet nerf, Mysterious Figure has (very) slightly less ridiculous damage output.
No Heart is pretty much a walking tank with insane HP and absurdly high defense. Rhythm Mixer is pretty much mandatory if you want to beat him in a reasonable amount of time. I imagine he's easier in multiplayer, since he and Armor of the Master (who's actually pretty bearable solo) were designed with that in mind.