|Lyn has traditionally been a unit known for her blazing speed yet underwhelming and fairly conditional offensive strength. With her Sol Katti refined however, Lyn unleashes latent potential to become one of the most powerful player phase threats in the game. The reason for this lies with a number of extremely powerful synergies that her reforged sword allows her to easily take advantage of. Once she does, Lyn can destroy nearly any unit in the game that she initiates on.
Weapon: Sol Katti with its refined Special skill is Lyn's weapon of choice. Once Lyn reaches 75% of her maximum HP, she is not only able to immediately make her follow-up attack after her first attack, but she guarantees a follow-up attack against any unit regardless of a difference in speed. This means that nearly any target that Lyn initiates on has to weather both her attack and follow-up before they're able to retaliate, a unique combination of skills that forms the basis of Lyn's offensive potential.
Assist: Reposition is always a versatile and strong skill that enables your team to move across the map quickly. However, if you want to activate her kit without risk, Ardent Sacrifice is recommended. The 10 HP it costs to use is enough to send Lyn into the activation range of her weapon and all her skill in most cases. There are however, some crucial exceptions: if your Lyn is merged or is unfortunate enough to have +HP as her boon, 10 HP is not enough to naturally activate her abilities. This is part of why HP is greatly preferred for her bane, as the threshold in which Ardent Sacrifice consistently activates her skills is quite high (the limit with being level 40+7) whereas Lyns with banes in other stats can only consistently activate through Ardent Sacrifice without any merges at all.
Special: Moonbow is the most consistent out of Lyn's choices, its short charge duration allows Lyn to always activate it on her first initiation. However, if you want to take full advantage of Wrath, Luna is recommended as the Special charge reduction at the start of every turn allows Lyn to consistently activate it every turn after she's reduced to 75% of her maximum HP.
Alternatively, you can use the Heavy Blade 3 seal in tandem with Infantry Pulse 3 and allow Lyn to instantly charge Aether or Galeforce with her first strike. This massively frontloads the first attack she makes on a map at the expense of her consistency in any battles afterwards.
A Skill: Brazen Atk/Spd 3 is the core synergy in this build and what gives it so much of its power. The 80% maximum HP threshold complements Sol Katti's requirement almost perfectly, giving her +7 Atk/+7 Spd in tandem with Sol Katti's effects. This transforms Lyn's somewhat modest offensive stat line into something truly fearsome and empowers Sol Katti significantly. Brazen Atk/Def 3 is an alternative, and might actually be preferred if you expect your Lyn to be directly in the fray often. That said, the primary reason that Atk/Spd 3 is valued over Atk/Def 3 is the prevalence of Swordbreaker skills among many units Lyn might face. Swordbreaker and the guaranteed follow-up that Sol Katti provides cancel each other out, meaning Lyn has to naturally out-speed a unit with this skill to get the full effect out of her sword.
B Skill: Wrath 3 is heavily preferred for this slot. Because Wrath activates at the same threshold at Sol Katti, Lyn receives a more pronounced benefit from this skill than many other units. The Special charge reduction and most importantly, the additional +10 damage it adds to every activated Special pushes Lyn's offensive capabilities to their absolute limit.
C Skill: As with most units, this is very flexible depending on what you feel that your team needs. If you want your Lyn to play somewhat more selfish however, you can take advantage of her fairly above average Res and give her Def Ploy 3. This softens up most units who might otherwise be too durable to fall after Lyn's two attacks.
Sacred Seal: Attack +3 is the best overall choice, helping her secure kills against tougher enemies. Heavy Blade 3 in conjunction with a more impactful Special can also be useful if you want to significantly accelerate her first strike on a map.
This build is somewhat sensitive to nature, but if you either have a boon in +Atk or a bane in -HP, it should function as intended to varying degrees of complete effectiveness. Though of course ideally, you'd have both. -Spd while the least desirable bane other than -Atk is workable largely because the automatic follow-up that Sol Katti provides does not rely on speed at all, though it does mean Lyn loses against more units with Swordbreaker equipped.
This build of Lyn has some notable weaknesses. Her naturally low defenses and being required to use some of her HP to activate her skills means that she's extremely susceptible to dying on the enemy phase or even the rare opponent who can withstand her opening attack. This means you should try to make sure Lyn is not within range of another enemy once she's finished with her target, or have a nearby ally ready to Reposition or Dance her to safety. As such, she doesn't fare well against Horse Emblem without significant terrain advantages because of the lack of breathing room or set-up time Lyn needs to truly thrive. And while extremely uncommon on defense, the Hardy Bearing seal knocks the foundation of this build from under it by preventing Lyn from immediately taking her follow-up attack against its' user.