+Atk-Def or +Atk-Hp Lucina?

I guess it mostly depends on the build. HP banes are usually preferred for Fury -> Desperation builds if memory serves right.

If you're using Falchion healing, keep her HP.

If you're not expecting to get hit by a brave weapon (including Meisterschwert), keep her HP because she's pretty glass cannon-ish anyways and I wouldn't expect her to survive more than one hit unless you use brazen atk/def, in which case drop her HP. But she should probably focus on her spd, not her def.