if you're asking about how to beat her, you bring Rinny then send her away when prompted, you then meet her instead of Pixie while doing the challenge and you can beat it, then have Marin do the challenge (normal or futa), she looses no matter what and it is currently repeatable (you have to beat the challenge again, then she loses again and so on)
if you're asking how to beat the second challenge forever and move on to the next challenge - there is no next challenge (at least so far), and the second one is just repeatable. it doesn't grey out in the logbook because of a bug, but it is finished for the time being