Instructions
Transfer the Pokemon you want to level onto the PokeWalker included with your copy of "Pokemon SoulSilver." Each step taken with the PokeWalker equals 1 EXP, so the amount of time it takes to level will vary based on how much experience your Pokemon needs before it reaches the next level. After you get the necessary number of steps, transfer the Pokemon back to your copy of "Pokemon SoulSilver," as a Pokemon can only level up once per trip. You can transfer the Pokemon back as many times as you'd like per day.
Put the Exp. Share item on low-level Pokemon, then have higher-level members of your team battle high-level wild Pokemon. The split EXP will help the low-level Pokemon gain experience quickly without putting it in danger of being knocked out.
Battle the Elite Four once you've earned all eight badges. Before you challenge them, stock up on healing items. The Elite Four have high-leveled Pokemon that give a lot of experience. Even if you don't beat all four of them and the Champion, your Pokemon will still gain EXP and can level up quickly.
Head to Mount Silver after earning all 16 badges. The Pokemon just outside Mount Silver are in their lower- to mid-forties, while those inside the mountain range from the mid-forties to the low-fifties. Watch out for the trainer Red, who resides at the top of Mount Silver with a team of Pokemon in the upper eighties.
Use Rare Candies to instantly raise your Pokemon's level by one. These items are rare, so only use them on Pokemon you really want to train.