|This build is intended for a flier only team. As usual, hone/ward/etc. C-skills are a must, and given the movement potential and dance support Ninian will provide I've found this makes "defeat so-and-so with a flier only team" challenges a breeze.
Dance. She's a dancer... Use it.
Blárblade+. Within synergistic teams blade tomes are always an obvious choice. Given her low Atk stats this is the best way to ensure your blue Ally slot isn't wasted on a support only role.
Atk/Spd Bond 3 is one of my favorite relatively new A-skills introduced. It is not turn dependent like stances and blows, and within a synergistic team your units typically stay near or next to each other. +5Atk/Spd is very significant, maximizing on her already solid speed and helps make up for her naturally low attack.
Luna is a great special because it allows Ninian to make up for her low Atk, If you anticipate a higher numbered turn battle, Moonbow is a good choice as well, sacrificing a bit of power for more frequent use.
Chill Atk 3 is the best skill for her, since it's free and already very useful.
Atk Smoke 3 is also my favorite C-skill due to the defensive capability it provides. Pairing Ninian with some allies with reposition allows her to attack, weaken the resulting enemies, and move safely out of combat range. If you think about it in a slightly abstract way, it increases each allies def/res by 7... So why not?
Flier Formation 3 maximizes on her support role. It increases her movement range for attacks as well as rescue dances so its an easy choice.