Lyn build

Well that depends as there are a few differences in her options. Fury and L&D are her best A slots (defiant attack is only good once low and panic effects ruin it). Since you have high speed Brash assault is not recommended but something like Escaoe Route for teleporting is very recommended.

L&D will give your Lyn 49 ark and 45 Spd. It will require a physical attack of 55 to one shot her and 61 on magic (assuming both are res sources) and the HP seal raises both requirements by 3 if you feel that is cutting it too close against some meta choices.

I like to keep galeforce myself and I use Savage Blow to help weaken her followup targets as well but spur effects are good with her teleport or threaten defense to secure even more kills works as well.

Fury will end up at 47 Atk and 42 Spd but will be far safer in terms of bulk, needing 63/69 power to one shot her on red at the cost of some offense and the recoil can help make sure she ends up below half health for her sword and b slot skill of choice.

I tend to really like the optimal build listed here which involves Defiant Attack and Brash Assault. Those two abilities gel really well with her weapon's inherent Desperation 2.