Okay, the more I think about it the more I like the idea of someone who's never defeated Cat defeating someone who defeated Cat when the survival of the world's on the line.
David A LudwigThe most likely scenario where Jasna defeats the Big Bad wouldn't deviate from canon until Cat and Melody face said Big Bad. If Jasna knew going into the fight that this was someone who defeated Cat, that'd give her the sense of urgency to go all out from the start. Which would be her best chance.