Hwayoung's S3 description reads as though it is supposed to be used to one-shot high hp units, however she still one-shots low hp units, making it so the best way to survive her S3 is to have even more hp than normal, which is a bit silly in my opinion. I fon't think it is fair that she gets to one-shot anything she wants with a skill that has such a low downtime.

I would change her S3 so that it didn't one-shot low hp units but still one-shots high hp ones (max hp, not current hp%). This can be done by lowering it's base dmg but increasing the dmg increase proportional to the target's hp (from up to 100% dmg increase to up to 200% for example, but with a 40% dmg reduction on base dmg).

Unlike other units with similar passives, her S2 gives 50% increased atk (Senya gets 30%). I think this makes her too easy to build when you then consider that she also doesn't want hp (but is compensated for it with a 30% crit dmg reduction), making the only stats she wants SPD,ATK and DEF. A unit being easy to build isnt really an issue, but when she has decently optimized gear she becomes too strong.
I think she is too tanky for a unit that has a 3 turn CD one-shot button that also cleanses and grants immunity, I would like for her barrier to have a lower scalling or for her dmg reduction to be lower.

Lastly if her S3 isn't changed to not one-shot low hp units, i would like for her S1 to not give her CR, she is already being built high spd, she doesn't need help lapping, especially with such a low CD on her S3.

Edit: Forgot to add that her S3 CD is way too low, could easily got +2 turns considering that her S1 gives her CR to help cycle it faster