+spd -atk Sumia

By Naga's Grace we definitely need Iv rerolls now.. her Atk is horrible but I always have a build for those in need.

Firesweep L+
Reposition
Galeforce/ Aether
LnD/ Brazen Atk&Spd/ SS/ Fury
Chill Spd or Def
Flier Buffs/ Spd or Atk Smoke/ Atk Wave/ Flexible
+3 Atk seal.

Since she has mediocre Def and Res, she should be able to tank some hits, even with LnD equipped. Reaching 49 Atk with 15 Mt from Firesweep L+, LnD and +Atk 3 seal should deal a bit more damage, now and with 48 Spd, but with Brazen Atk/Spd she can reach 51 Atk and 50 Spd, Who do you think will out speed her?

What I am doing for mine with the same ivs I am keeping her base kit as her lance if a foe attacks first shes given +6 attack. So for the seal give her Attack +3 and refine her weapon to be attack as well so when a foe attacks her the attack with go all the way up to 50. Plus giving her luna will help quite a bit and with good enough defense her close defense skill will protect her well enough so she can attack right away with her high speed.

Though mine is more if you wish for more of a budget build.

I don’t see how this would work. Sure 42 speed is decent enough, but 27 atk is literally horrible. That’s 41 with your standard weapon... This just doesn’t seem usable. At least she’s nice fodder with Close Def 3

Fodder is definitely an option. She's teeming with gold.

If you don't want to fodder, she's usable. You're looking at a myrmidon-ish statline with the low atk and massive spd, which means special-reliant builds tend to work well. For example, harmonic lance + moonbow. But you'll need to figure out how to enable her to keep firing moonbows once she drops below the HP where she can take another hit. One idea is to refine her weapon for def and use brazen atk/def, and someone else on the team has ward/fortify fliers. Now she has 40-46 def once she drops below 80% HP. Ideally she would be able to use

The other thing you can do is play the Selena game. Use brazen def/res (keeping close def should also work, but you'll need them to initiate into you for max damage), Berkut's lance+def or harmonic lance+def, guard (to prevent them from charging their specials while you charge yours), something like chill atk, and stack another close def on top of it all. And get some ward or fortify fliers support. Charge up an ignis or glacies, and unleash a 30-40 damage special on someone.