You can get away with almost anything, if it fits the plot. Or you fit the plot to it. But fortunately, the plot is demanding a Deadly Encounter for the party -- large though it is, it's going to survive because of a character's knowing the crime lord's weakness, and the crime lord's assuming that the same character can't overcome her own weakness to exploit that knowledge. That narrows down the choices. Obviously class-based.
I think I'm going to make her a sorceress, and one whose spells concentrate on the offensive and on light and fire. Also a physically enormous, powerful and intimidating figure -- hmmm -- not a half-ogre because that would bust the world-building -- a beastman of some type. Possibly lion to make her bright and sunshiny in appearance when she's a crime lord.
Plus some powerful objects, because this is a high magic world, and her foes have them, too.