I think I did a monotype once, but generally I don't go for the 'official' challenges (by which I mean all these ones with their own threads). Instead I just try to limit myself - pretty easy as I'm quite weird and obsessive - in a lot of small ways that add up to make it more challenging, such as: only using Pokémon introduced in that region (only really a limitation in Diamond / Pearl tbh); only using Poké Balls introduced in that region and the standard one (great fun in Johto, but only with a [I]lot[/I} of clock manipulation); only using items introduced in that region; and finally - one I won't repeat - forbidding myself from grinding, only battling trainers, which led me to attack Hoenn's Elite Four something like 20 times before my team were tough enough.