It's not so much that they're significantly tougher in the way of strategy. There's just a huge jump in levels from the eighth gym to the Elite Four. It kind of sneaks up on you. First you're fighting stuff in the low 50's, then you're up against a team with nothing below 60.

I've decided to just pound out some levels and use brute force and then pick up the last five pokemon I need for the Sinnoh Pokedex. After that, I'll migrate Charmander, Bulbasaur, Squirtle, and Eevee to abuse EVs with. =D