I'm personally enjoying Johto story, leveling up my pokemon is never be a problem eventhou need more time to get high level.
If you want to beat it by your own, try to use exp booster. Rods have a decent varieties of wild pokemon and level as well. There's a chance if you grind long enough to level up, you'll meet a rare or even a shiny pokemon, that's a quite advantage. If you still feel bored, no worries, you can buy high level Johto pokemon from someone to easily beat the story, no need to get decent nature and iv pokemon with super high price, any nature and iv of high level pokemon will do. Or, if you have friends already beated Johto, just ask them to lend you one or two sweepers. Good luck :Crazy: