Find Steven in Granite Cave. (you don't need flash, but navigating the cave might be kind of annoying without it) After delivering the letter to him and getting the TM for steel wing, go back to Rustboro City and talk to Mr. Stone, and he will give you an exp. share.
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Put your low level Pokemon in your party's first slot and give it the exp. share, and set your battle mode to shift. Each time your opponent sends out a Pokemon, switch to your low level Pokemon, and immediately switch to a higher level Pokemon. Your low level Pokemon will gain about 3/4 of the total experience from most battles.
This works best if you do all of it before entering the Dewford Gym, because then you'll get a lot of experience from the gym itself, but you can get the exp. share anytime after delivering the letter.
