How should I build a Fae +spd/-atk ?

For now, Fury, QR, anything in C, and Iceberg. When you merge her a little more and get better IVs, you can invest in better skills. At that point, you can either stack speed, use Wrath instead of QR and sport Spd Ploy (and also use the obligatory Steady Breath) or balance between speed and defense and use QR, Atk Ploy. The former is more glorious. The latter helps teammates better in dragon emblem teams. And there are also light breath DC builds which I don't really think is as good as Dark Breath on Corrin.