I was thinking about Geometry yesterday because HK had mentioned not knowing how her daughter goes from knowing nothing to everything on a subject. It was in freshman geometry when I figured out how my brain did that stuff.
For a topdown/abstract thinker all these facts come in like random cards in a card catalog. Until the index system is identified and understood, that information is inaccessible. Once the brain works through some models and hypotheses and some obscure point of balance is passed this whole scaffolding pops into place and all that data is accessible.
Geometry rewards a very topdown perspective with bonuses for visualization skills. To my memory 90% of year 1 geometry can be derived from Pythagoras' Theorem.
I think the best way to have challenge is to throw out the motivation of satisfying the school's requirements and to rather drive for one's own complete understanding while encapsulating the inadequate expectations of the class.
