+ATK -HP Lucina

Both her Defense and Resistance are pretty low as is. Giving her Life and Death would only serve to make her even more frail. I'd rather go with Fury.

For her B slot, Desperation or Swordbreaker would be a good idea.

Her C slot is pretty flexible, but I would go with Threaten Speed or Defense.

Her assist should be Ardent Sacrifice, and her special could be Moonbow, Luna, or Draconic Aura. This is just my opinion though. Hope this helps.

Since you have -HP I would indeed go with Life and Death. This has perfect synergy with Ardent Sacrifice (which gets you go 75% HP) and Desperation (which activates at 75% hp).

If you prefer going Swordbreaker, I agree with IronButterfly that you should pick Fury. In this case I would choose Reposition or Reciprocal Aid for my assist though.

IronButterfly already gave the best advice for C slot and Special. Good luck, and I'm so jealous of your pull! x)

Personally I run fury/sword breaker/spur atk with swap and moonbow.

You can give her the hp+3 sacred seal if you want to fix up her hp stat. Make sure you give her desperation and reciprocal aid though.
She can take a hit from blue units with hp that high and if it's a physical unit she will probably kill it due to her 58atk and 41spd. After she has her desperation active.
I find reciprocal aid better than ardent sacrifice and because you can merge her and switch over to healer later on.

I had fury on my +atk Lucina, then switched over to LnD. It works much better for desperation build imo.