I've encountered what I believe to be a bug.
This guy (requires Rock Climb to reach) only battles you if you've defeated all trainers on route 11 and as I battled a trainer with the same sprite in Crimson Volcano (requires Strength to reach) and received a mega stone I assume that will be the case with this one.
I made a full circle around route 11 including checking out its other Rock Climb areas and I have not found a single trainer that I did not defeat the first time I went through it to go to Tarmigan Town but this guy won't battle me. He keeps telling me to defeat all trainers on route 11.