Which Ninian for dragon team?

+Def/-Atk, Ninian isn't much of a heavy hitter and the extra def should help more that her lack of atk.

+atk -spd isn't good, she won't be seeing much combat (and she's no powerhouse either way) and -spd means she will get doubled a whole lot more than she would like.

+hp -res is not preferred as it takes from her ability to tank.

If I were you I'd go with +def - atk, like Prefils also said. She's a support unit who enjoys allround defenses and enough speed to prevent doubles.

Dark breath is fine as she will have more options when debuffing, Fury 3 will also round her stats up some more. As someone who uses her on various teams (dragon enthusiast), i actually recommend non-ranged Ninian. This way she can lure ranged units whenever you're in a tight spot.