Really? I don't remember RD:RoL to be very hard. I had a pretty easy time Nuzlocking it. Although now that I think about it, I DID farm for a lucky egg. Having a Lugia for half the game kind of kills the difficulty, though.
Otherwise, I'd have to go with either Bloody Platinum or Manly Pink. The former is just made to be absurdly difficult, but then the champion has a team made of ubers and you want to be sad. Manly Pink, on the other hand, just has horrible balancing issues. Also, the level "curve" would be more accurately described as a level "cliff".