|Obviously, the main Goal with this Build is to make use of Mia's Speed and her special A-Skill Flashing Blade 3 to make her one of the very few characters who can use Galeforce effectivly in Battle. In order for this set to run perfectly, i highly recommend to use her alongside Dorcas with his Infantery Pulse.
The idea is very simple, yet effective: Mia will start every battle with Galeforce on 3 cooldown (2 if you use the Killing Edge+), so it will trigger on her very first turn if she
A: Doubles her opponent
B: Get hit by a counterattack
C: If you run Killing Edge+, Galeforce will always trigger as long as you have 1 Spd more then the opponent.
Here is the more in-depth description:
Her best IV is obviously +Spd in order to trigger Galeforce more reliably, an Atk+ is also good of course.
For her bone, i would recommend -Res as she should do her best to survive at least one counterattack (expecially in Arena Defense)
For her weapon i recommend Resolute Blade, because usually you will trigger Galeforce on your first move even when you start with cooldown 3. Although it does NOT trigger its in-built Wrath effect, it still provides the most power because of its boost of +3 Atk on top of its 16 might and Mia's extremely high Attack. If you dont happen to have a Dorcas although to reduce her cooldown on turn 1, maybe Killing Edge+ is the prefered choice. With its might of 11, it should be enough to kill most enemys in the 2 turns you can attack.
Reposition because why not
Obviously we take Galeforce as her special, because thats all this set is about. If you might need just brute might in some situations however, you can think about using Astra instead, as in combination with her Resolute Blade it completely demolishes almost the whole cast with one single blow.
Flashing Blade 3 is essential in this build as it allows to charge Mia's special in one turn if you are faster than your opponent (which is all this set is about, triggering Galeforce as soon/often as possible).
I recommend Desperation 3 for her B-Special, because if you manage to get in the HP-treshold for Desperation, Mia just turns even deadlier, often OHKO her opponents without being threatened by a Counterattack.
Here you could play around a little bit tho, as Swordbreaker 3 or even Lancebreaker 3 MIGHT be an alternative, improving her survivability in some cases.
Now, the C-skill is rather interesting. You can go for usual team boost skills like Hone Atk or Hone Spd, but in my opinion, expecially if you want to use her in an Arena-Defense team where the only goal is to eliminate only 1 opponent, Savage Blow 3 is a very interesting choice as it damages opponents other than the initial target (which could come in handy if she then tries to kill another opponent than the first one). Another idea is to play Attack Smoke 3 to lower the opponents offensive options after her combat, although in my opinion it is not worth to sacrifice a Brave Lyn for this.
And last, but defenetly not least as it is crucial for this build we have Quickened Pulse as her Sacred Seal Special. (I don't think i have to explain this one)
The last thing i want to talk about are Mia's "optimal" teammates, or at least some very good options for her.
I use of course Dorcas, as he reduces her Special Cooldown by 1 on the first turn. Again, if you are not lucky enough to have a Dorcas, you should use the Killing Edge+ instead of her Resolute Blade.
Second is basically any Dancer, although i prefere Performing Arts Azura (with Wings of Mercy 3 ) as she provides her allies with buffs while she dances.
As her third ally, i came up with Trick-or-Treat Nowi with a very simple idea: i wanted to have the highest and most unpredictable movement options possible.
For this, my build is a Warpgate Nowi with Grimoire to warp to allies 2 spaces away when above 50% HP, Wings of Mercy 3 to help out allies in trouble and Guidiance 3 to let the other members of the team get nearby her and maybe secure a kill (which again is the only real goal for an arena defence team)
I hope my explanations were clear and understandable :)