While I can see this happening, I get the feeling that will be Izanagi with his sword vs Izanami with her sword....which mean Ragna and Noel vs Izanami and Nu. To be honest I hope i'm wrong on this.
Then again, I wouldn't mind if Nu become good.
Easy to say "destroy her" but is really the right thing to do?
Just think about the story that Rachel has told them and what the hero has always did.
Also there is that scene when Ragna attacked Izanami and Saya words, not to mention of what Izanami said after that.( I remember someone translated this part)......what I mean is: I get the feeling that Saya (and maybe Izanami) is that girl.
Anyway , sorry for my bad english, I'm still learning.