-Def or -Res L!Robin?

Depends. When in doubt keep her phy bulk intact due to so many armors.

If you need her as a DC units with Dd, -def is better.

If you need her as a melee unit with close def. Then -res is better. If shes getting doubled by a dragon then shes probly ded or on low health eitherways.

Definitely depends how you plan to use her. I have +spd -res and I only use her to tank mages in a pinch. She'll do it as long as she doesn't get doubled, which shouldn't be too hard with a flier buff. -def is a bit more balanced; it also leans toward desperation builds.