Suggested build for S!Lucina

36 neutral speed is nothing to laugh at and +Atk will still serve her well. In a flier team, one hone flier buff will make her at 38/42 atk/spd. To make the most use of the owl tome if you plan on using that, she should always be adjacent to at least one other unit and will grant her more stats in combat. With Quick Riposte, she will always make a followup attack if she's healthy enough. This has great synergy with the owl tome in the enemy phase. You should use Moonbow on her so if the enemy initiates on her, she gets +1 charge then she counters with the first attack for another +1 charge. At this point, Moonbow is ready. Because Quick Riposte allows a guaranteed followup, she makes a 2nd attack with the special and will likely kill the target. The result is also the same if her speed is naturally higher than the enemy even if you do not have the HP requirement to activate Quick Riposte. Just keep her Swift Sparrow until you get the chance to put Close Counter on her so she can deal with both melee and ranged enemy initiation with the above strategy. You can wait out on her C skill until you have the resources for it. In a flier team, any flier buff would do nicely. In a mixed team, Guidance or Threaten Def/Spd can work too.

if you dont feel like using many units you can always make her into a pseudo healer + buffer. upgrade her egg tome to +spd, give her breath of life seal, upgrade her rally skill, give her a hone skill, and maybe some type of sweep skill so she doesn't take damage when healing up her allies.