And you definitely DO NOT need level 70 Pokemon to take down any Elite 4, even FR/LG's second version of them o.o
If you want an easier ride through them, try to be level 55 or above with all six of your Pokemon. That said, I made it through with Pokemon under 50 and, while very tough, it's doable with the right amount of heal items on hand.
Buy lots of revives and heal items, ideally Full Restores but Max/Hyper Potions and Full Heals will help get the job done, too (not to mention that they're cheaper).