Basically, it's a challenge where you have to go an ENTIRE pokemon game, only leveling up one pokemon. Sounds hard, doesn't it?
I started this challenge two days ago in Crystal, using Typhlosion. I'm currently at Ecruteak with a lv.37 Typhlosion and five useless HM slaves.
Sounds like fun
I did this a while ago in a red rom with charizard. Beat the elite for at ~level 87
I tried doing this before, won't work so well once Double battles come in, the twins on that bridge don't challenge you if you only have a Treeko in your party.
Though I did it for fun and didn't save.
This is how I played Red version anyway when I was 6. I figured why train more than 1 guy if the 1 guy is really strong. =P
I've heard you can beat R/S/E using JUST Mudkip/Marshtomp/Swampert (as in, he IS you're HM slave as well as fighter). That'd be insanely hard, though.