Wasteland Knight
Adventurer
I’ve played in three campaigns over the years where the characters were at or above 20th level. I can’t imagine a mountain climb being any sort of challenge for characters of that level.
Think bigger. The mountain IS an earth elemental. And a terrasque lives in its armpit.b) Massive Earth Elementals (think The Hobbit)
Here is the bad news: Not all game challenges are level appropriate across every single level. In fact, I'd say that it's far more likely that a challenge type does not span every possible level, at least outside of class challenges. Climbing a mountain is an extraordinary accomplishment at 1st level, some place just above 0-level Commoner.