Best build for +Def/-Res Ninian?

Light Breath is fine. Dark Breath is arguably better, but it shouldn't matter too much, since Ninian should mostly avoid combat. Either way, refine it for speed so she'll be doubled as little as possible. Luna is her best special since her stats are too low to use anything else. Fury is a good A skill for general use, because it raises all her stats and her HP is high enough that the residual damage won't affect her much. She shouldn't be entering combat more than once or twice anyway. Triangle Adept is also an option if you just want her to specialize against reds. Wings of Mercy is the classic B skill for dancers, because it lets them warp around the map and refresh injured allies. Windsweep, if you have the fodder, is another option that will let her apply her weapon buff or debuff without retaliation. You can give her basically whatever you want for her C skill and seal.

The main thing is that you'll want her to survive one or two combat rounds. Ninian is support, not a fighter.
Get her speed to at least 36 with refine/A slot/seal you could boost her def/res a little too but avoiding doubles is the most important thing.
I wouldn't go for TA for A, with fury or some stance skill she could take a hit from a green unit.
For B wings of mercy is probably the best.

Here is my build for -atk one, she is really nice mixed tank especially if she isn't doubled. After one round her stats go up and she becomes even more tanky.