Well, of course it's going to be Shinku who wins, inevitably. Making Suiseiseki win would be too unpredictable. But lets not factor in the author's intentions. Just consider her combat abilities and willingness to fight.
Sure, she hadn't been a warrior in the past, but now that Souseiseki is gone, she might be somewhat irked. And this could give her a reason to use her abilities to oppose Suiseiseki aggressively, even if only to take back Souseiseki's Rosa Mystica.