The more I think about it, the more I think Jihl would have been done better if, instead of Barthandelus killing her, he had made he a l'Cie with the Focus of killing the heroes. Telling her just enough truth about them, that their Focus is to destroy Cocoon, to motivate her to do her best to kill them, all with the intended purpose of using her as a tool to make the heroes stronger in the long run. Maybe the first fight, Pre-l'Cie, she takes them on in a mecha. Second fight she starts in a mecha, then after they bust the new model open, she pops up and reveals her new l'Cie powers, maybe even summons a hostile Eidolon. And for her final act, have her realize her master is the true enemy, only to be killed by him when she turns on him. Sort of the noble in the end type enemy, who really does believe she's fighting for the right side.