Lyn

This build's goal is to get Lyn below 50% health. Once she is at this threshold, she can initiate a double hit, along with a wrath empowered moonbow hit on basically anyone in the game. Even without the brash assault seal, her 42 speed with life and death is enough to double and kill almost every ranged unit in the game.

Thanks to the new Weapon Refinery System, Lyn can become a solid unit with high speed and good offensive prowess. Though there are definitely better units, this build works for all the Lyn Fanboys (including myself). The new effect allows Lyn to not need the Brash Assault Seal and use the Attack and Speed +3 seals. Swift Sparrow could be swapped with Life and Death 3 depending on if you don't care about preserving her bulk or Fury 3 to speed up the time to activate the Sol Katti's effect. Wrath can also be swapped to Vantage 3, however personally I'd run Wrath to ensure that she hits hard. No stat is high enough for a special based on stats, so go with Moonbow cuz every other skill is TRASH (Thanks Abdallah). Threaten Def is also good to ensure more powerful strikes. The Seal could be either Attack 3 or Speed 3. With Speed 3 it acts as if she has plus speed IV and Attack IV. Or go Attack for even more power. She becomes a sold offensive force and easily her best set.

A build that I believe might end up taking full advantage of Lyn's build. Now that the Brash Assault seal has been released, Lyn can finally shine!
Sol Katti's effect is Desperation 2, meaning at less than or equal to 50% HP she's able to do an A-A-B attack sequence when initiating combat. Judging by how many builds Lyn has here with Brash Assault, I'm sure you know what that does — but I'll explain anyhow. Brash Assault 3 allows the unit to make a guaranteed follow-up attack when less than 50% HP and attacking a foe that can counter-attack it. Combined with Sol Katti, this means that at less than 50% HP she's guaranteed to trigger Desperation when she initiates. However, there's a catch...due to the fact that Brash Assault had to use her B slot, she didn't have any other options in that slot (like Drag Back or Escape Route 3).
Not anymore! Now that Brash Assault is a Sacred Seal AND the recent update now allows you to upgrade them to 3, anyone can use this classic combo with Lyn AND be able to use a B skill! This is where Escape Route comes in, as its requirement (having less than or equal to 50% HP) is already being met by Sol Katti and Brash Assault, and this handy skill lets her teleport adjacent to any ally as long as this HP requirement is met.
Now that I'm sure you all understand where I'm getting at, it's time to move on to the other skills. Fury 3 is used to help get her HP down into the above skills' range while also giving her +3 to her battle stats. Galeforce gives her the ability to act again once a turn if she triggers it, giving this already insane build the ability to shut down multiple units with ease. Her C passive is versatile, though if she's using Galeforce it might be a good idea to have a Smoke skill (Spd Smoke 3 would be the best one, as it may allow her to Quad more easily, and I'm pretty sure Quad-attacking with Desperation is a death sentence for any opponent).
In the end: she can teleport adjacent to any ally and slaughter her opponent, then (if she has Galeforce) she can teleport again and slaughter another opponent. Tl;dr: I introduce to you, the Lyn Reaper!

Budget 50% HP Lyn build. Brash Assault suits her natural kit well, especially Sol Katti's Desperation 2.

I do want to replace Defiant Atk 3 with Death Blow 3 so I can have an attack buff above 50%, and still be able to receive Hone buffs below 50%. But she's not on my priority list.

Sword Demon of Sacae

This set take advantage of Sol Katti inbuild Desperation and grand her major boost on offensive side which she usually lacks. Turning her into "crit machine"

Furry3 gives he better offense as well as helps her get into desperation/Wrath range. With combination of these two skills she can trigger Moonbow in every battle (and add 10 damage flat to it) she double enemy which is majority of roster, especially with speed boon giving her edge even against many blue enemies.