I really had a problem with that particular scene. I don't see why Souseiseki wasn't able to take her down, when Shinku was.
And she died so quickly against Bara Suishou later after that performance. With Suiseiseki's Rosa Mystica. I personally believe that either
A. Souseiseki should have destroyed Kanaria during first encounter
B. Kanaria should have matched up with, or even destroyed Bara Suishou during their battle.