Spring Lucina

While I can't confirm nor deny if there's a conspiracy at Nintendo HQ to keep Lucina down (perhaps they fear her great might? perhaps...), I can definitively say that with great investment you can *force* Spring Lucina to be viable.

Weapon: Flora Guide (+Spd) or Spooky Censer (+Spd)
Flora Guide is the closest we can get to a spd/res lull Blue-tome weapon at this time, which is why its chosen. Assuming you take at least 3 damage from your first round of combat, then Fury 4's recoil will put Lucina in both Brazen and Desperation range.
If you desire a slightly more consistent build or one that is more suited to surviving enemy-phase attacks, then Spooky Censor will guarantee you get below 75% hp after one round of combat and the additional bulk helps to survive attacks and avoid ploys.

Assist: anything you want. Just remember that Arena scoring values the advanced stat boost assists (e.g. Rally Up Atk+) to a noticeable degree compared to movement assists.

Special: Ruptured Sky, Blue Flame, or Galeforce
Ruptured Sky is chosen because its the shortest cooldown special that scores highly in Arena calculations. It consistently gives an extra 8-14 damage which is perfectly serviceable. Blue Flame has a slightly longer cooldown but its damage is far superior & more consistent, but requires Lucina to be next to an ally (which, while uncommon, can lead to troublesome situations).

A: Fury 4
Because of her BST, she really needs this. Lucina's below-average HP actually helps here as the recoil damage from Fury 4 is *almost* enough, by itself, to get her within that 75% threshold to activate Brazen Atk/Spd and Desperation.

B: Desperation 3
As much as I would love to put Lull spd/res here, desperation is simply too vital as Lucina relies on this to survive engagements against meta heroes that can counter-attack.

C: Anything you desire
Pulse Smoke, Joint Drive Atk, Joint Drive Spd, and Time's Pulse are all very useful options, but what is best will depend on your team.

Sacred Seal: Brazen Atk/Spd 3
+7atk/+7spd = ouch, especially since Fury 4's recoil helps to get Lucina in this threshold.

[Her tome is actually Juice Wave+ with a Spd refine, but that's not an option on the builder for some reason.]

Bunny Lucina makes up for her terrible Atk stat with her staggeringly high Adorableness stat. Sadly, this doesn't let her hit any harder, so it has to be patched up in other ways.

Juicy Wave+ is an expensive sacrifice, as it's on another seasonal unit (Summer Camilla, specifically) but it works well here and is a worthy investment. As Juicy Wave+ has the same effect as Desperation 3, it frees up Lucina's B Slot for a different skill.

Ardent Sacrifice is her best Assist skill, as it immediately takes her down to Desperation and Brazen range with the bonus of healing an ally for 10 HP.

Brazen Atk/Spd 4 is a very expensive skill, but if you're using Juicy Wave+ then you've already sacrificed one summer unit to a sub-optimal spring unit; what's one more? It's the most synergistic option available, as dropping her health by 10 will be enough to activate both Brazen skills and Juicy Wave+'s Desperation effect. This will grant Lucina +16 Atk/+17 Spd on both phases and the ability to strike twice consecutively on Player Phase.

(Brazen Atk/Spd 3 is your best option if the next level up isn't available, giving Lucina Atk/Spd+14.)

Lull Spd/Res 3 allows Lucina to cripple her foe's defences, all but ensuring that she achieves a follow-up attack and increasing the amount of damage she'll inflict, two things she desperately needs to do in order to be effective in any way.

(Chill Res 3 is a cheaper option that will still give Lucina a much-needed boost to the damage she deals to whichever foe is affected by the -7 Res penalty.)

Even Atk Wave 3 is a great choice of C Skill for her, because Lucina wants all the boosts to her Atk that she can get. Lucina will need at least one turn to either take damage in combat or use Ardent Sacrifice to drop her health, so she generally likes an Even skill better. A Spd Wave skill is also an option, but once both Brazens are active she's hitting 52 Spd even with a Spd bane and no merges; neutral IVs or a Spd boon push her over 54 Spd, so even with the speed creep Lucina will probably need an Atk boost more.

Finally, Brazen Atk/Spd 3 for her Sacred Seal, because Lucina needs to stack Atk Brazens to have a threatening offensive presence, and she wants lots of Spd to ensure that Juicy Wave+ is useful.

Based on a summer Ylgr build I ran into. The concept is Special Spiral+Flashing Blade resets Miracle after every combat, and the Egg tome (or any of the spring weapons) heals a little after combat so that Miracle can recharge.

• Preferred IV: +ATK or +SPD / -DEF
• Weapon: Spooky Censer+ (+Spd)
• Assist: Reposition / Draw Back / Rally Skill / Player Preference
• Special: Luna
• Passive A: Life and Death 3
• Passive B: Desperation 3
• Passive C: Time’s Pulse
• Sacred Seal: Flashing Blade 3

(+ATK, -DEF)
HP 37
ATK 52
SPD 46
DEF 17
RES 23

Spring Lucina’s offensive stats is lackluster, but is decent enough to actually be viable. With that being said, she will be needing all the buffs she can get to actually be able to KO an opponent in two hits, so support units such as Ylissean Travelers Olivia, Performing Azura, Legendary Azura, Soiree Berkut are examples of some of the support units that can dance and give stat buffs due to their prf weapon.
If possible, Legendary Eliwood, if you’d run a team focusing on Spring Lucina, because you can’t have an ally on a team with a higher Attack stat than Spring Lucina’s (aside from Legendary Eliwood himself). If you do, the ally with the highest Attack stat will get the Bonus Doubler effect of Ardent Durandal.

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Spooky Censer is a good weapon for this build. It has built-in Fury 3, which gives her +3 in all stats (besides HP) and deals 6 damage to herself after combat, which is pretty useful considering this build is about making follow-up attacks with Desperation. With that being the case, you don’t actually need a set-up to let her reach Desperation range.

Luna is her special because when partnered with Desperation, Time’s Pulse, and Flashing Blade. At start of turn, when cooldown count is at it’s maximum value, grants -1 Special cooldown count. Automatically making Luna a 2-cooldown count Special.
When initiating combat at Desperation range and Flashing Blade in effect, she gets to charge Luna at her follow-up attack, usually leading to KOs.

If you do not have a Time’s Pulse fodder (Sothis, at the moment of writing), then you should run a Special with a cooldown count of 2, such as Moonbow, or Glimmer. Replace the C skill with anything, preferably self-boosting, such as Wave skills (Atk or Spd).

With Juicy Wave+ (spd), spd seal and swift sparrow, Lucina will initiate attacks with 54 speed. If attacking an opponent that has been chilled, there will be an additonal 6+, essentially allowing her to double almost all foes. After one attack, she will also be able to use the innate desperation function of the Juicy Wave tome.