How does new Sol Katti actually work?

2, Brash Assault doesn't mean you quad if you automatically double anyway

basically if youre below 75% then you do 2 attacks in succession and that's it. so basically shes slightly worse elencia

The most you can hit someone is only ever 4 times in a single turn of that singular character. And that is a scenario where the unit is fast enough to double using a Brave weapon. Anyone other then a Brave weapon can only ever hit twice on their own.

High speed units may naturally double most others, but Brash Assault allows it so you may double them regardless, which is nice because there is a lot of high speed units being used;even more so true with these new sword units.

So long as she's below 75% she can duel characters like Ayra and Mia, though I still think she needs her special at the ready.

Personally, I would say it is very useful. Try putting Vantage in the B slot...watch her double first counter all the peoples. Except of course, when Hardy Bearing is in play.

To explain Brash Assault Unit automatically makes a follow-up when at HP ≤ 75% and attacking a foe that can counter.
Means if the Enemy can not counter you will not make a follow up short said spd rule come back.
Brash Assault can break enemy rules like Wary Fighter and the normal spd rule get applied.

Best IVs i would still say +atk for more dmg.
Her spd should be high enough to defend her vs enemy follow ups still is a safer option ofc still hurt her overalldmg that she needs because of low atk stat.
37 neutral spd thx with a slot /buff /spur /seal / summoner support and new system with Blessing(still not always up depends on week) can be managed to block atleast always enemys follow up.
28 neutral atk is her critical stat as a dmg dealer that why i say atk boon is more important for her.

+Speed is still a great IV, though +Attack is generally better. If you're fearful of not being able to double units like Ayra or Mia, Phantom Speed Seal is great for securing those. Wrath and Moonbow pair phenomenally with Lyn. Swift Sparrow is pretty much her best A Slot, only slightly outclassed by Heavy Blade/Flashing Blade and Luna, but those are too rare and better served up to other units who lack the same kind of A Slot optimization.

I think now Lyn have a niche as she is the only one being able to go with Brash assault + Desperation + Wrath allowing her a 100% sure proc of moonbow with good damages