Just to know that I'm on the right track.
My pokémon were all lv65-70 when I beat the Elite Four and it was a piece of cake. I didn't even have to grind much to get those levels.
My Pokémon were around 62-65 and I was just fine. You should, too!
My Pokémon were around levels 70-75, and if you used the new EXP Share the whole time, it should be no problem to get there! :)