The best training place in Emerald is to continually battle Gabby the Reporter and Ty the Cameraman. This is a double battle that consists of Magneton and Exploud. You first battle them on Route 111. You then find them on Route 118, just across the water from Mauville. After that they will move to Route 120, east of Fortree. Your first Poke is a strong one with STAB Earthquake (Swampert) holding Amulet Coin or Lucky Egg. Your second Poke is the one you want to train, holding an Exp Share. Your third Poke should be one that flies or has Levitate, holding Amulet Coin or Lucky Egg.
You attack with EQ and then switch your training Poke with your third Poke. EQ should OHKO Magneton and typically Exploud, if you trained your first Poke correctly, missing the third Poke you switched in because it flies or has Levitate. Battle over. Now, since your training Poke had Exp Share, it will get 75% of the Exp Points earned. Amulet Coin will double the money you get. Lucky Egg gives the Poke holding it a 50% Exp Point increase. So, as you can see, you can level up a Poke rather quickly since you can continually battle them, following them from route to route until they will finally stay at Route 111. You can also battle more efficiently since when you battle the E4 you have to go through all 5 of them. Here, you battle only as many times as you want. Simply the best place to train ever.