what to do with -atk b!ninian?

B!Ninian is a dancer first, and a fighter second, so her main role (usually) is providing support to the team. With a res-refined Fresh Bouquet, and Fury 3, she hits 33 resistance which, in my opinion, is enough to run Ploys, like Spd Ploy and Atk Ploy to help your allies tank hits better.

You can also make her into a Bladetome unit, but with her stats and role on the team (usually support) it's not really something desired.

You can make her into a mage tank with Blarserpent and Distant Defense, but you'd be giving up some support (replacing Chill Atk with something like Quick Riposte) to do so.

She's a dancer so her attack isn't typically her focus.
In spite of the attack bane, you can still keep her useful.

She can be a great flier buffer if you give her Fortify Fliers and a Hone Attack Seal which combined with her weapon, means she gives +4/+4/+6/+6 which is great for bladetomes or any unit in general.

I personally made mine a bladetome unit which even with your -atk can work if you provide enough buffs through someone like Eirika/Ephraim or a Hone Fliers.

Finally, she can also just be a typical dancer with Wings of Mercy and maybe Triangle Adept where she'll function just as good as any other dancer.

If you're concerned about the low atk and you need her to do some combat, you can just give her TA and be a dedicated red counter.

her atk is already bad so its not much of a problem except for her lowered bst because its a super bane but that only matters in arena and by no means must you take a ranged dancer into arena unless they're a bonus unit

Either give her Triangle Adept and just be fine with hard countering reds, or go full Res and give her Blarserpent and double DD; her Atk was never really salvageable to being with in my opinion lol, and hey, I would take a -Atk B!Ninian anyday! (on my 3rd pitybreaker sigh)