Faye

This build is focused on making Faye impervious to magic-- when attacked by a ranged unit, she has a whopping 53 res. Warding Breath is chosen over DD3 as an A skill because of the special cooldown charge.

This build makes her very very tanky, guarantee +18 DEF/RES, plus her basic 26/31 def res will make it out 44 def and 49 res when attacked without buffs or spurs.

Most of the mages and archers will 0x2 her if you buff or put her into a fort and since she is really slow (even with SPD bane or not) together with QR3, will make her trigger Glacies faster.
(If you get lucky and get hit by some x4 attack archer, you can Glacie and kill him on the same turn)
Glacies should do around 39 fixed damage, enougn to kill most horse units and good damage on others, even on armors.

Swap/Reposition skill works well on tanks, you can always save your units from the enemy range with it.

Ranged tank Faye build, you pretty much win every single ranged matchup if you get engaged on.
+Res/-Spd, but any -Spd IV works, Faye doesn't need speed. Res is most useful because of the Ploys.
Alternatively you could take Guard as B skill, or DD3 as seal.

IVs: +HP/Def/Res/Atk, -Spd

Yet another obscure counter for the infamous duo of Rein/Lyn comes Faye, who shines as a defensive archer with a solid defensive spread 42/26/31 at neutral. While she pays for this in 25 Spd, this problem can be mitigated with breaker skills.

The main gist of the build is get as much bulk as possible to take on the duo: Spd is not an issue when facing Reinhardt, and Lyn outspeeds near anything anyway, so Bowbreaker is taken for Faye to be able to tank and defeat Lyn on the counter. To survive either of their onslaughts though, Faye requires the combination of Distant Def 3 for her A slot and from the seal, pushing her defenses to 38/43. Finally, her C slot is the obvious Panic Ploy given her high HP of 42, though various Ploys such as Def Ploy or Atk Ploy can be used instead if one wishes. It should be considered that debuffing Reinhardt or Lyn will contribute much to Faye's ability to counter them, and will also allow her to shut down other bladetomes.

Slaying Bow is taken with Glacies for the damage spike with her Res, especially on the counter, to help secure kills especially given potential Fortify Boosts reducing the damage she can do when countering or attacking, however this is relatively flexible. Her Support Skill is otherwise flexible as well, and in this instance Reciprocal Aid can be taken if she needs to tank multiple powerful ranged attackers. Finally, her IVs are also wonderfully flexible as well, and anything that reduces her Spd so as to avoid tampering with her bulk and offense will work fine.

With weapon refinery, Faye has multiple options to boost her stats. Her defense and resistance will generally appreciate a boost of +3 and +4 respectively, with the former being especially important against all variants of Brave Lyn, especially with defense being the lower of the two defensive stats.

Using my IV screwed Faye, (+spd/-atk) I have made a build that can, with an Olivia glued to her side and the occasional speed boost, kill nearly everything not running Wary Fighter. Including literally any Hector variation without Wary Fighter.

That was the goal of this build, btw. Killing Hector. Killing everything else was just a really, really nice bonus.

Downsides: Lots of investment in a unit most think is mediocre at best. Olivia is the main drive behind her power and she can be more effectively used to boost an already powerful unit. Most matchups are decided by who attacks first.
Upsides: You kill 99% of your opponents with a single unit.

IVs: +spd/-whatever. (You'd probably be much better off without my -atk lol)
Weapon: Firesweep Bow+.
Special: Luna. (Iceberg can be taken, but it's far less reliable in this build and won't always ko Hector, especially since we're lowering our res. Moonbow is even less reliable and won't ko Hector at all. Don't take Moonbow.)
Support: Something to support her allies. Can't hog all the glory. Maybe Draw Back to keep your Olivia safe.
A: Life and Death 2. (Most matchups can be won even without L&D 3. It's only a few that you need the extra 1 attack and speed. Without -atk, you'd be fine. With -atk, L&D 3 is sadly necessary to secure some kos. rip my 20k feathers you will be missed lol)
B: Seal Defense 3. (Poison Strike 3 is another very solid option. This is what really enables her to get with the murdering. If they attack her first and she survives, she gets even more use out of it and may not even need a Dance to ko in a few cases.)
C: Threaten Speed 3. (Sometimes a speed nerf saves her. Or lets her secure a kill she wouldn't normally be able to, like Sharena.)
Seal: HP. (Enough defensive matchups are close enough that those few extra HP save her from certain death.)
Teammates: Olivia. Somebody with a speed boost. Somebody who has a good enemy phase so you don't get stomped. If your Faye is -atk, somebody who can deal with M!Robin and F!Corrin.

An attack boost and an extra turn is all Faye needs to ko all but TWELVE neutral def/hp units without Wary Fighter. A speed boost brings that down to EIGHT. Those eight, being Y!Tiki, Lukas, F!Corrin, Arthur, Saizo, S!Camilla with Gronnraven+ and Triangle Adept, M!Robin with Triangle Adept, and Subaki with Iote's, can be dealt with by her teammates. A speed boost is needed to secure kos on M!Corrin, Merric, M!Robin (except Triangle Adept), and Ogma.
Letting them attack Faye first to proc Seal Defense lets her ko Y!Tiki, Lukas, Arthur, IS!Subaki (thanks to Threaten Speed), and Saizo, bringing the total of Do Not Engage units down to THREE. (F!Corrin, M!Robin, and S!Camilla) All of which can be dealt with easily if your Faye has neutral attack and you let them get the first hit! (Be careful if they have a +atk nature. It could lead to your death.) (Neutral attack 5*/+atk 4*+0 Olivia can take out S!Camilla for you if Faye is -atk.)

My current team:
Faye (+spd/-atk)
Olivia (+hp/-def) (Dance/Moonbow/Fury/Renewal/Hone Attack)
Nowi (+spd/-atk) (Rally Spd/Moonbow/TA/QR/Threaten Res)
Julia (+hp/-def) (Reciprocal Aid/Draconic Aura/Fury/G Tome Breaker/BoL)

Tips:
On the enemy phase, if she's in danger, she survives a surprisingly high number of attacks. Though, anything with a Brave weapon besides Draug will absolutely murder her, so be sure to stay out of range.

Common units like Linde, Lucina, and Reinhardt with Death Blow will double and kill her if you let them.

As a general rule, anything with a decent attack stat that can double Faye will be more than capable of killing her if you let it get a chance. Which is why Threaten Speed and speed buffs are such great things.

Be sure to always check your matchups and enemy buffs before engaging. A stray defensive IV spread can ruin your deathless streak if you're not careful.

For even more death of your enemies, you could totally run two/three dancers if you wanted to literally only rely on Faye and don't care about BSTs or losing horribly on the enemy phase.

Nothing with Wary Fighter can be easily killed since you aren't running any breaker skills. Kite them to death as usual. And don't let them touch you; the Seal Def proc won't help and Effie especially can kill you if she has a Brave Lance. You do survive a wack from +atk Hector.

Iote's Shield won't help any fliers but Subaki survive due to their generally low defenses. Even +def Beruka falls thanks to her low speed.

Try to make Aegis, Holy Vestments, and Sacred Cowl proc on a hit Luna doesn't. They will often save the enemy's life.

Bowbreaker is usually your worst enemy. If you can't get rid of it in one hit, Do Not Engage. Use your teammates.