Narma hands down.

Frozen banshee offers poison, poison sensitivity, and turn meter boost. Narma, offers poison, poison sensitivity, Atk down, secondary heal, and debuff extension, along with having better stats than FB. The only real advantage fb offers is that she's easier to book.

What Carpe said, except of course that the books part is a pretty huge factor. Narma fully booked would be a CB goddess, but who has that many legendary books to spare? And if it's a booked FB vs an unbooked Narma, I'm not so sure if Narma is still so much better, especially if you have a counterattack or ally attack team that can get extra A1s from FB. 

My current Clan Boss team is a Counter atk team:

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You do absolutely need an attack down, whether it's Narma or somebody else. Get rid of Elhain who isn't doing anything useful there, while Athel and Warmaiden are also pretty meh on CB and can be replaced by better options. 

Athel is, of course, not top at CB, but she is not that bad. She offers weaken and has an A1 that hits three times, what brings, with Giant Slayer masterie, more damage than a single hit with Warmaster.