The Mc has always know how to use Qi? He knew how to use Qi but got buffed further by that one reverse trap girl. Then he became better after the mountain, where he learned quite a bit.
I know which one u talking about now. It's where he escorted that one girl that he met from Mush Mountain and encounter with that one "chimera" girl. It should be during the Kin Arc, probably chap 68-69.