Ninian Build Ideas?

Unless it's a dragon team, go classic dancer WoM build.
For training... triple healer?

I gave mine Triangle Adept and Wings of Mercy, and Moonbow for her special. I also had her eat a bad 5* Nowi for Lightning Breath+.... I like Ninian.

For training her, unequip Dance and send her to the training tower with a buffer (Eirika works best), another dancer, and a healer. My go-to is Azama, because he can easily weaken dudes without killing them.

Lightningbreath+ is the way to go for her weapon so she can devote her turn on dancing while allowing her to dish out damage in enemy phase. For special I'd run moonbow since she'll ever attack in the first place. Otherwise Bonfire is okay as well, although that will be hard to trigger unless you have quickened pulse seal.

Dragon team:
A: Fury or TriAdept
B: Swordbreaker (needs it to survive Falchion users)
C: Fortify Dragon

Standard dancer:
A: Fury or TriAdept
B: WoM
C: Hone/Fortify ___ or Panic Ploy

Tips on training Ninian (& other dancers in general) - unequip your dance so that one of your other dancers can dance/sing for you. Healers with Pain staff will help chip down the enemies regardless of their defense without actually killing them. Hone Atk on the team will help secure some kills as well.