The challenge you're speaking about is who has the power in the relationship. As in "the one who cares least, wins". There's no reason to think of it as a competition if you're confident with yourself. You don't have to be more of a challenge than her. You know your value and that the more a girl knows you, the more she'll be attracted to you. I'm not always available, but i'll change my schedule to see her if I like her, and I expect her to make as much effort as I do. If she doesn't, then she's gone.
My interpretation of challenge is to simply not give her anything that she hasn't earned. That includes gifts, my time, my affection, commitment, insights into who I am,even compliments. If she can't take anything for granted, even the next date, then she realizes she has to work to please me, and has to pusue me through her actions.
From my attitude, it's clear that I don't need a girl. I'll be aggressive sexually, but in a way that shows i'm not pressed for it. If she pulls back, I take it in stride, pull back, and then try again on my own timeline. I never ask a girl why she took so long to call back, or why she didn't do something she should have, because I don't really care. I simply move on with my life, and make her earn her way back into my good graces if what she did was forgivable.
This doesn't change, no matter what the girl's schedule is. Her method of being a challenge doesn't factor into my actions at all. I do the same thing, whether she's in love with me or barely interested, whether she's a "10" or a "6". Without any words, girls learn what my limits are. They don't make the same mistake more than once. The ones that can't meet my expectations get weeded out, stress-free.
SCARILY familiar.