Challengingly means in a way that is hard, but in an interesting or enjoyable way — the kind of hard that makes you want to try, not the kind that only shuts you out. A teacher can set work challengingly: just above what you can already do. A climb, a puzzle, or a piece of music can be written challengingly, so that getting through it feels like a win. Though actually, the same adverb is also used for a look or a tone that pushes back, as if the person is daring you. That stretch is less “fun-hard” and more “I am not backing down”. It is a fairly careful, written sort of word; in speech people often just say “in a challenging way”.