-Def/+Att Lucina how should I build her

+Atk Lucina is not bad. Just give her skills to ensure that she destroys whatever she attacks so she doesn’t get countered too often and lose too much health. :) I don’t really use her so I don’t know what to suggest for builds...

But I think she’s worth building.

It’s worth it with those IVs. It’s not a perfect IV set but it’s a pretty good one. Give her the “our bonds give me strength” effect refine for Falchion if you can. Then use Fury/Desperation or Swordbreaker(this is mostly for coverage against Ayra/Mia). You can also use Distant Counter, but imo there are better users for such a rare skill. That’s not to say she can’t use it, though. In this case you should use Quick Riposte as B skill.

You can also use Reciprocal Aid and Renewal to make her a medic.

With +eff Falchion she reaches 46 HP, so she can be a Panic Ploy/Infantry Pulse user in case you need one.

Totally worth it. The easiest build is the bruiser build. Assuming at some point you have the Divine Dew available, definitely refine her Falchion. Even if you can't, bruiser build boosts her stats by going Fury as the A-slot, and either Vantage or Depseration in the B. Depends on your preference - I prefer vantage. Ardent Sacrifice can also put her in range to activate the B skill. C-slot is flexible depending on resources. Drive skills, Infantry pulse or even just her Spur Atk are all decent options. Sures up her defense and capitalizes on her attack boon. Renewal 2 on her normal weapon also offsets the damage a bit. Very potent offensive build.

Combat medic Lucy is the other main choice. Focuses on healing other team members with renewal as the B slot and reciprocal aid as the support skill. Can keep Fury on her if you'd want, but any of the Atk bonds work well in the A slot too for that roll. Also consider giving her a rally skill for the support slot and you have a potent medic who can dish out damage and heal for a ton mid-battle.

Either build is very effective and I still think she can be as good as Mia or Evil Celica with investment. Ayra is Ayra, but Lucina is far better than many credit her for, and with Dragons in the meta she can be an absolute monster. Enjoy my favorite girl!