I find Noire's and Gerome's paralogue the hardest (that Ive done). With Noire's being worse. And Legault, its not just about her magic. Her durability is like, worse than Miriel, so she wont be having a good enemy phase any time soon. This will in turn, affect her exp gain.
EDIT: Severa's is also somewhat difficult but Ive managed to do it right away in Chapter 14.