The best way to level up is to battle Rogue Cail. You can only do this
after you have Pyrite City on your map and after you Snag Cail's Shadow
Pokemon. After you beat him, (his Pokemon are on level 40 after the
first battle), you should save your game and heal your Pokemon and leave
town. Then you come back and talk to him. You should switch Espeon out
on the first turn or it will get knocked out by Granbull's Strength.