Title says it all, can you post a link to a good challenge. I just beat the game using only my starter at around level 70... IT was the easiest thing ever, even easier then having more then 1 pokemon... So any challenges?
Huh, I like it. Why I haven't thought of a speed run I will never know, I do those all the time!
And as for the Nuzlocke challenge, it seems like it only adds unnecessary rules to the single pokemon game. With only using your starter, you have no need for other pokemon, you don't ever use items or TMs, and you rarely fail at one-shotting everything in the game... But I will try and add my own rules to it.
You're honestly just better off finding a more challenging RPG. There's not much challenge to be had unless you're actually battling with someone else in real life who can put forth unpredictable or strong teams; the main game just isn't all that tough.
Something like Final Fantasy Legend II/SaGa 2 (probably cheap on eBay) has more replayability in the main game thanks to choice of team having a huge impact on gameplay, and the game itself is more challenging & satisfying to complete than Pokémon, which allows you to basically just dump EXP into one decently flexible Pokémon and use its massive levels & items to roll through the game.
The main problem with challenges in this game is that they are more of a test of patience than a test of skill. Try a no healing challenge though. It still requires luck, but it's not as bad as a Bulbasaur minimalist run, and it's not too luck intensive.