I have two Lucinas and I'm struggling to choose what to do with them. One is +Res -Spd, the other is neutral. I was leaning towards a healer role to help with the upcoming Tempest Trials and assist my team in general, but it seems like a waste to not utilize her for battle. I'm f2p, so please no skill sets that have rare skills I can't obtain.
I would say go with neutral. Having -spd might hurt a bit and res is one of her dump stats.
My Lucina is built with Fury, Renewal (2 works just fine but 3 if you have it), and has Reciprocal Aid as support...I also like Aether for the extra healing but it's up to you. I've used her in both Tempest Trials and she's awesome. One of my best units. Even with a healer build she does quite well in combat as long as you're careful about the Fury drain.
IMO if you want to make use of both Lucinas use the -spd +res IV Lucina as a healer for tt.
Build:
Falchion
Reciprocal Aid
Luna
Fury 3
Renewal 3
Hone Atk / Spd or Spur Atk / Spd 3
As for the neutral IV Lucina for general use I would build her something like this.
Build:
Falchion
Reposition
Dragonic Aura / Luna
Fury 3
Desperation 3
Threaten Spd 3
Pokemon GO
Fire Emblem Heroes
Fate Grand Order
Dragonball Legends
Dragalia Lost
IV Calc
Heroes