I read that the Battle Frontier is complete, aside from the Battle Pyramid and Double Battles in the Battle Factory. Does that include the Challenge Championship? Can you face off against all the Gym Leaders, Elite 4 and Champions?
yes you can. It’s been like that for a while
You couldn’t battle the Elite 4 the last time I challenged it, except Lance because he also qualified as a Champion, and there are some trainers like Blue who automatically kick you out of the challenge before you can even battle them.