A skill for fallen Robin?

I’ve got the same nature and I’ve given him distant defence to make him a ranged wall. However, if you don’t have that skill to give or need something more budget friendly, death blow or fury could be viable, whilst atk +3 or def +3 are both cheap and will supplement his 2 biggest stats

I'd give him a breath. Warding Breath with the Distant Defense seal.

distant/close defense with the opposite as the seal
Warding or steady breath work but i'd only reccomend it if you give him aether.
if you have none of those fierce stance works
none of those still then fury or +3 res/def whatever you want him to tank more i guess
OR triangle adept if you want him to delete blues, depends on your team

Steady Breath is optimal for fast special procs, but I'd recommending swapping B slot for Bold Fighter if you're going down that route because the CDR effects do not stack for Vengeful Fighter. Otherwise close/distant def is a good alternative but limits your Robin to an enemy phase unit

Ideally (and specially with those IVs), Distant Defense.

That is, unless you decide to go mixed phase, in the case you want Warding Breath, Bold Fighter, Armor March and QR seal.