Elite 4. I always use a Lucky Egg and Exp. Share if I can't beat E4 yet with that Pokemon.
Well, I do it like this. Beat E4, Fly back to Pokemon League, Beat E4, Fly back to Pokemon league, rinse, wash repeat. That's what I did on Emerald/Sapphire/Ruby. But now, I just ask for others to use Rare Candy on it.