'I have the power to lift a mountain, but this is unexpected...'
Since he learned earlier that he can't cheat through the process and jumps up to the peak of the mountain, Rex is now climbing the mountain like a regular man. With his kind of strength, it should've been really easy to climb a mountain.
Even though he doesn't have any experience in rock climbing, his strength should make up for it.
But contrary to his expectation of easily climbing to the top, Rex finds that there's a huge pressure pressing him down from above. Due to that pressing force, climbing the mountain is hard enough that he actually needs to hold on tightly to his grip.
Rex's arms are bulging with muscles, he gazes up realizing that he's not even a quarter of the way.