If you want Lucario before the Pokemon League, then put Riolu first in your party. Fight a Pokemon that is many more levels stronger. As soon as the battle begins, switch into a Pokemon that can defeat the wild Pokemon. Riolu will gain many, many EXP points, and probably end up approximately LV. 15 already. If you are patient and wait until the Pokemon League, do the same there. It may give a disadvantage winning the battle though. If you are aven MORE patient, you can wait until Stark Mountain, there the Pokemon are as strong as they get in the Sinnoh Region. For memory, around LV. 65s!
Just remember, Riolu evolves through happieness in the day, so make sure its happieness is high during the day, and avoid it fainting; this decreases happieness. Bitter roots/herbs also don't help at all. Winning battles and items other than the bitter stuff help increase happieness. More happiness increasing things: