Selena

Boon/Bane: +Def/-Res
Weapon: Armorsmasher+ (+Def) / Armorsmasher+ (+Eff)
Support: Reposition
Special: Ignis
A Passive: Steady Breath
B Passive: Wrath 3 / Guard 3
C: Threaten Spd 3 / Threaten Def 3 / Threaten Atk 3
Seal: Quick RIposte 3
Summoner Support: No
Ally Support: Someone who can deal with Blue units, especially Blue Tomes or Brave Lances.

Anti-Hector Selena. Either upgrade of Armorsmasher+ is good enough, though going with the +Def version adds some more physical bulk (putting her at 39 Def with a Def boon). Adding Steady Breath's +4 Def during the Enemy Phase and you're looking at 43 Def just by her own power. With Ignis, 43 Def translates into 34 damage...and with Steady Breath's super-charging effect combined with her 35 Spd and Quick Riposte seal, you should be activating Ignis every battle she can counter. Wrath is there to increase her damage and charge time when she falls below QR's HP requirement, but Guard is a safer alternative when up against offensive units.
Her natural Reposition and Threaten Spd are perfectly fine, though Threaten Def or Atk are good alternatives if you want her to have a little more offense.
Unfortunately, her Atk is quite pitiful and even with Armorsmasher+ and Wrath she needs at least 2 attacks to finish off a Hector regardless of phase. While she's very fine as a wall, she won't be doing much of anything to non-Armor units outside of her Special activation. If you're going to use this build, be prepared to have other units take offense..especially against Blues that would tear her apart.

I made this build to take advantage of Growing Flame's huge AOE radius. This build works well on any bulky infantry/armored melee unit, but I chose Selena due to her decent mixed bulk and access to Wo Dao.

Wo Dao - To add on to the AOE Damage that Growing Flame does.

Reposition- Default Skill

Growing Flame- For doing lots of AOE damage.

Steady Breath- Speeds up the charge of Growing Flame. Fury or Close Def work too.

Wrath- Speeds up the charge of Growing Flame/Adds to its damage. Vantage also works.

Threaten Atk- Lowers the damage she takes. Savage blow works too for added AOE damage.

Close Def SS- To boost defenses. Savage Blow and Quickened Pulse also work.

Selena is, in my opinion, a very underrated sword unit.
She has a really good stats spread, which cover defense as well as resistance without damaging her offensive stats.
Her best build should be the one I put up there, however I prefer Sword Breaker instead of Wrath because 1) it's easier to get and 2) it gives selena a better matchup against faster sword units.
Just for the sake of it, i'm gonna say why this build works well:
Weapon: Slaying Edge+ (+Res) it's her best choice since it has a -1 on skills charge and it gives her a good synergy with Fury & Wrath, however Wo Dao+ is a good alternative if Slaying Edge+ isn't accessible.
Assist: Reposition is her best choice, like for almost anyone else in the game, but if arena points are needed a double Rally can be played instead.
Skill: Moonbow is the one which sinergizes better with her kit, she gets a -2 skill activation and +10 damage wheb <=75% health thanks to Slaying+Wrath, but if arena points are needed Aether can be run instead.
A slot: Fury is her best, nothing more to say; 3 more points to everything are good and -6 hp every fight is good aswell since Wrath gives her benefits from it, and her general bulkiness can mitigate most of the damages that enemies can deal.
B slot: Wrath sinergizes too much with her kit too give up in it, however if Wrath isn't accessible SwordBreaker is a good replacer.
C slot: Def ploy is her best choice since she has 35 def and will proc it on basically anything that isn't an heavy armored unit, if not accessible C slot can variate basing on your needs.
Seal: Atk Ploy is the best choice since, like def, she has 35 res, and will proc it on a lot of enemies without too many problems.
I also wanted to point out the fact that with summoner support and a +10 she reach 43 Hp, 51 Atk, 44 Spd, 41 Def and 41 Res, which are abnormous for an Infantry Sword Unit.

This is my second of 2 builds for Selena. This one is a lot more affordable, but I use this on mine, and the results are incredible. I would highly recommend this build if you want a perfect physical tank who can deal out 45+ Damage with ignis (mine is at +2 and with a +4 def buff she deals around 50 damage with ignis).

This is my first of two builds for Selena. This one is the more expensive one, but it seems like a very optimal thing for her if you can afford it. If not, check out my other build for her which is much more affordable.