How do you build Ninian?

bond skills and stance skills all work as replacements. As long as you run light breath or lightning breath you should be ok.

I use mine as both a Falchion counter and res tank. She's +hp -spd, but it'd probably work with any ivs.

You could put either triangle adept or spd+3 on her.
These might seem like wierd choices, but:

TA makes her a good red check, this allows you to build the rest of your team more freely.

Spd+3 is good if you want her to survive. With it and a boon, she'll have 39 speed.
Adding to that; she'll have 67 physical defence and 74 magical def. (if you have a refined breath she'll have 47 health and 20 def/27 res). Do note that brave physical attacks only need 44 might on neutral.

Otherwise you could always give her fury, that's always a viable option

Here's my cheesy offensive Ninian.

life and death or fury might be useful her
mine is fury, WoM, and fortify dragons, and she serves pretty damn well on my dragon team