Help with IVs for L! robin

I would probably go for the -def one. I really have a thing against atk banes, but that's just me. I guess it depends on her teammates

I don't know if 26 defense (def js a superbane) on a colorless enemy phase unit would be a good choice. You need every single point of defense on Robin that you can get, because you can't abuse the weapon triangle with her.

The same can be said about attack (since she can't abuse the weapon triangle to boost her dmg), but even with the bane it's enough to kill ranged units if you use quick riposte and melee units usually fall down thanks to bonfire. She can have problems with armored units tho (and anti-dragon units ofc), so make sure that you have someone on her team, who can deal with those.

It'd be better to choose the -atk version. I'm using my +res -atk copy as a flexible tank in a flier team and she works well, because she's one of the few flying units who can tank in the enemy phase without needing lots of expensive skills inherited.
I know that -atk sounds awkward, but it's definitely not the end of the world and I think that keeping the defense of a tank unit neutral is a higher priority.