Should fodder my +res -Atk Silvia or build her as a res tank

Build her, She can be a pretty good mage counter/Support unit(with ploys).

I don't recommend Special spiral on her, it will be better on a more offensive unit.
You could just go with the generic build and give her fury+Wings of mercy.
BUT
since she has pretty good res you can go for Barrier sword(res or def refine, go for def if you want her to survive physical hits better.) Glacies, DC,WIngs of mercy,Ploy skill,Ploy seal or Distant def.

Build her. Trash her Atk entirely by giving her double Fortress Def and the Def refine Barrier Sword

Her attack sucks anyway, and as a dancer she probably shouldn't see combat anyway. Make her a res tank