How would you build a -spd +def Legendary Robin?

def boon isn't my preference on her, but pretty much any boon is good on her. With -spd I don't think you get away without QR unless you buff the hell out of her in a flier team, but in that case you're better off buffing some Cordelia or blade tome through the roof.

Once you give her QR she should be fairly good as an anti-physical unit, both ranged and melee. Guidance is fine; you could use the distant def seal to really bulk up.

If you want to be really, really safe, give her guard and use the QR seal. She will become extremely difficult to one-round. I prefer this treatment for A.Tiki because she's slow slow that she gets doubled by everything and we only get one QR seal each. With L.Robin I suggest you give her some speed buffs from the team to block more doubles, and if you can, merge into one without a spd bane. My F.Grima has +spd -res so she can't tank magic too reliably (one hit is probably fine, but she won't have much HP left); blocking doubles eliminates the need for guard, which leaves the B-slot open for QR.