Lyn

This set allows Lyn to be a nigh unstoppable unit on player phase. +Atk -HP is the desired Boon/Bane on this set as it allows her to more easily put herself under 75% health with Ardent Sacrifice, but almost any bane is okay as long as Atk is the boon. Sol Katti's refinement allows Lyn to double AND immediately counterattack again when under 75% HP as it is effectively Brash Assault and Desperation in one weapon. Ardent Sacrifice is the support skill as it puts her in range of her HP thresholds after one use. Brazen Atk/Spd is the best A slot for Lyn as it gives her +7 Atk and Spd when she's under 80% HP and it synergizes extremely well with her weapon. Wrath is the desired B slot as it gives her an effective special charge of 1 when under 75% HP and using Moonbow or Glimmer. Atk Smoke is a good C skill as it allows her to increase her survivability thanks to its effect but a hone spur of drive can be used in place if you desire. Moonbow is used as the special as with Wrath plus her weapon she'll be able to pull it off in every engagement effectively nuking almost everything bar extremely bulky blue units or units running Swordbreaker. Lastly Atk +3 is her preferred Seal to further increase her firepower.

An improvement on her "optimal" set, it trades Life and Death 3 for Brazen Atk/Def 3, which boosts her Attack even further under threshold, as well as giving her ample defense.
Of course, the reason Brazen Atk/Speed wasn't chosen is because Speed doesn't do much for her, as her weapon gives her Guaranteed followups and her speed keeps a good portion of units from doubling her anyway.
With Wrath, she can get off Moonbow every turn under threshold.
She boasts high res, but if Def Ploy isn't working often enough, or if you can't get it, use it for Threaten Defense instead

Another build giving a new breath to Lyn.

Since her weapon greatly shine when she's attacking, I focus on that : A player phase Lyn.

Weapon is straight forward, a Sol Katti with +eff boost her kill counts to over 90% (some strategy required)

Special is Luna since in higher ranks, there were many opponents with high merge. My +1 Lyn could not kill them with Moonbow alone. With Luna, even a +10 Effie could be another victim if Lyn is buff properly. She just need around 5 atk buff, and there goes all the hp in one battle.

Assist skill is reposition since she would be needed for moving people around.

A is a DB3 instead of Fury or L&D. The reason is that she would have to take some hit to reach 75% hp. Those 5 extra def / res could literally mean Life and Death. She do lose 5 extra spd, so unless she has a +spd boon or heavily buff, avoid tanking Mia or super speed Ayra at all cost.

B is a Wrath 3. The +10 dmg is good. But the actual usage would be for the special CD reduction, which would be heavily exploited by retreating the team around the map, until Luna is ready.

C is my presonal preference. Since she is stil low on attack against blue, the debuff would mean another +10 damage.

Seal is a +3 attack. QP seal could also be used. But its usefulness goes down to 0 after her first Luna.

With those setup, and some spur and / or hone attack, Lyn could reliably kill most units. Red, Green, Blue. Colorless is purely depends on her speed since her BA would not activated on those target.

She is meant to be under careful player control, so everything usable are used. Summoner support and Water Blessing boost her to 46/49/43/28/31, which is 55 attack before any other buff.

With said stats, Lyn could take a hit from most (selected) opponents, to activate her Desperation and BA, and switch on her killing mode.

So far, most reds are dead, unless they are super tanky, speedy, and using a sword breaker.

60-70% of blues are dead, except Nowi and Effie who could be packed with lots of hp, and might required some planning / buffing before engaging them.

Blues with Gems and TA should be avoided, except they are not tanky and Lyn has some buff to support her. Azura is one of such case.

Greens are not a problem. B.Ike is an exception since he could tank the second hit very well. Tanking him instead is a prefer choice.

Colorless are varied since Lyn might not be able to double them. Especially those tanky and speedy daggers, or a buff B.Lyn.

Stats include summoner support. Imagine ally support and buffs if needed.
Sol Kati refined with Desperation3 and Brash Assault 4 effect.

Very strong offensive setup with tanky bulk against def and res.
Just put her in front and let her clean the map if you take care of threats for her. I run her with a dancer to keep the galeforce working.

(Might go for damage Special with Wrath 3 but will rather use Wrath on a different hero as I like her Performance already.)

Seal and Special are really flex slots, but those two fit really well anyway so why not. But the main focus of this build is keeping Lyn's health as close to 75% as possible to enable as much death as possible. Quite insulting towards the enemy, with 53 atk on Player Phase before buffs she's incredibly deadly against any foe.