Best non-Aether special for a +atk -def fallen robin?

Bonfire works well with his great defences (even at -def). I guess the other options are glimmer or draconic aura which are quite powerful attack focused skills.

Ignis or Bonfire

If you really want attack, then i would suggest using draconic aura from f!corrin. If not, then ignis and bonfire are good. Vengeance is still good too, just a bit more situational

Ignis and bonfire will do less damage than dragon claw and draconic aura respectively. So I'd recommend draconic aura, wich has the same cooldown as bonfire

If you want the damage, choose Ignis or Glacies. If you want healing, choose Sol.

60 atk, targets lower of enemies def/res... id go for glimmer tbh