Share. To do this you'll need an exp. share. Here's where to find Exp. Share in the games;


Diamond and Pearl: South of Eterna city there is a cycling circuit, before you reach it though you have to go through a building. Inside, there is a professor who will give you Exp. Share if you have seen more than 35 Pokémon.
Ruby, Sapphire, and Emerald: Talk to Mr. Stone in Rustboro after delivering the letters to Steven and Captain Stern.
LeafGreen and FireRed: Go East of Fuschia and go into the building that connects the Fuschia to the route. Go upstairs and talk to the Professor's aide, he will give you Exp. Share if you have more than 50 Pokémon.
 Share to the Pokémon in the front of your roster. Once you have gotten Exp. Share, give to the Pokémon in the front to hold while battling.
 Beat your opponent and the Exp. Share will give not only the Pokémon that battled experience points, but all of the Pokémon you are carrying with you.
Find an Exp.
Give Exp.
Start the battle.
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