Mia

The stats above are being used are for an unmerged Mia.
Max stats at full Investment and weapon equipped would come to
Hp = 44
Atk = 57
Spd = 49
Def = 34
Res =31
This build means to capitalize on Mia's ability to inflict "True Damage" on her opponents. Her Weapon and refine already gives her +17 damage at max per special proc. Blue Flame increases that by 10 or 25 depending on if an ally is standing by and Wrath adds another 10 when the threshold is reached. At least, she'll be dealing +27 damage to her attack and at best itll be +52 no matter the situation. This build is very situational and is not meant for Aether Raids but for Longer Game modes such as Tempest Trials.
With all this stated let's break this down.

Her assist skill should always be a choice that reduces her hp and ardent sacrifice will always put her immediately into wrath's threshold. Through multiple trial runs I've found it to be the most reliable compared to reciprocal acid since Mia's Hp stat is low compared to other units.

Her special, Blue Flame, only comes from Owain and is a very high investment option. This special comes with obvious draw backs but with Mia's ability to stack flat damage from her weapon it's very potent on her. Obviously she won't have an ally around at all times but her def of 34 will typically allow her to survive a single attack from non blue physical units and she should hardly ever be doubled.

Her A skill being Swift Sparrow 3 reinforces her high speed and increases her attack but anything that increases her speed and attack is acceptable as a substitute. Alternatively, if she has access to buffs of some form, this can be switched out with def/res 3 for extra bulk as at max investment she'll be sporting 40 def/37 res but unless shes heavily merged this isnt an option.

Between the skill Spurn 3 and Wrath 3, Wrath is far more reliable at dealing damage while Spurn 3 is best for survivability. Both deal increased damage but Wrath's +5 dmg and -1 special charge are better for Mia's build in this case since all her dmg comes from her special proc. I would only recommend Spurn 3 if you're focusing on a more defensive play style as her nuke potential is already somewhat fragile.

Her C skill can frankly be anything but I run her on an infantry team so infantry rush has been working for me. Her and Soren especially make a very nasty duo.

Her seal I have as Flashing Blade 3 to help her charge her special. It's easily her best option, since her A slot is typically needed to increase her stats during combat and the extra 5 dmg from flashing blade 4 is negligible at this point.

Hardy bearing guards her from Vantage foes, especially useful for this build in particular where AoE special's main drawback is that it activates skills like Vantage, Brazen or anything thats an HP threshold.

This build capitalized upon Mia's base kit. As of 3.12.0/ 4.0.0 Mia gained a refined akin to Nino(YT)'s Gigs Excalibur, combing with Phantom Spd do deal with 70% difference upwards to +10 Speed. Meaning with SS PS3, Mia at -8 Spd above will deal extra damage on attacks. Reposition to allow her to use other Ally's as shields or to allow Mia to tank hits. Vantage 3 allows for first attacks when HP is 75% or less. Savage Blow for chip, And Phantom Spd 3 treats unit's spd as being +10 if they have Any skills that compares Spd. Stats for Atk, HP and Spd are +1 as My Mia this is based off of has 3/10 Dragonflowers. Noontime is used as with Flashing Blade, has a +1 Charge per unit's attack which means the instant she attacks, providing her spd is at least -9 to her opponent's will have Noontime ready to strike. Again My Mia has SB 2 and PS2 so it's -8Spd

Preferred IV: +SPD / -RES
Weapon: Armorsmasher+ (EFF)
Assist: Swap / Reposition / Pivot
Special: Aether / Moonbow / Galeforce / Luna
Passive A: Fury 3 / Fury 4 / Swift Sparrow 2 / Flashing Blade 3
Passive B: Null-Follow up 3
Passive C: Even Def Wave 3 / Even Res Wave 3
Sacred Seal: Flashing Blade 3 / Atk/Def Bond 3 / Close Defense 3

+spd/-res Feel free to go with another bane if you so choose, but after a merge it no longer matters. The +spd is rather important for any Null Follow-Up builds and with some armors being deceptively speedy, you willl want to capitalize on Mia's speed.

Resolute Blade really hurt to switch to Armorsmasher. Those 5 points of attack were sacrificed because it just was not killing armors enough (specifically Surtr and Legendary Tiki), even with the Wo Dao effect. When the meta shifts away from armors, you can change back to the more universal weapon.

Aether (or even Galeforce for you sly dogs out there) for arena, Moonbow or Luna for every where else. Aether and Moonbow are preferred because it will help in completely obliterating defenses, which is the name of the game for this build.

Fury 3 or 4 works well because Mia need all the stats she can get to overthrow our armor overlords. You are more than welcome to keep Flashing Blade on A and put an Atk/Def Bond 3 in the seal. However, that drops her RES and Mia tends to run out on her own, so having stats in Fury that she can take with her is hard to replace.

Null Follow-Up is obviously the key to the whole thing. A lot of people forget it has 2 effects. Not only does it stop Quick Riposte, Bold Fighter, and Vengeful Fighter, it also negates Wary Fighter, Great Flame, and the entirety of Swordbreaker. This leaves it to a 100% pure speed check which is why I gave it to Mia as very few people can check her speed. Throw in her very serviceable defenses and she is one of the top people for the skill in my opinion.

I feel that the Even Wave skills are great for your defenses and enemy phase moments because by turn 2, the enemy has arrived and you can adequately be buffed to take their hits. By turn 3, you are cleaning up with your player phase again. If you feel like you are good on Defense, then Even Res Wave is a good switch.

As is the case with any unit, so long as you reasonably build your team around her, Null Follow-Up Mia can take down the current meta easily.

My Mia is built on Speed. She will be doubling units a lot, and focuses on lowering enemy speeds for her benefit, and her teammates' benefit. Due to outspeeding almost every unit in the game, she will charge her special very often, and Astra will be ready to go in 2 hits. If she's under 75% HP on Enemy Phase, she can activate Astra and finish the enemy off without taking more damage.