For me, approaching the level 40-50 mark, I had to start doing two brawls to kill her. At first I just switched my crew around just to do the 1000 or so HP that remained, but as I got closer to 50 I had to start using half ATK filling potions to get her.
I forget how much damage I did every time I swung, but the last few times it was over 100k damage. Before hitting 50 I was doing around 70-80k. I can attest that to having four Sashas before the event due to a lucky (?) 11-draft before the event even started, giving me a huge head start with damage multipliers.
With the changes in ATK regen, this event is only going to get harder. It was fine before when I was waiting to kill the little bit of HP that remained after a swing, but now that I have to wait three times as long, no way in Hard would I do what I did when the event started.
Approaching 60 I had to start using full force twice in order to bring her down. With the changes to the ATK regen I'm definitely stopping where I'm at. It's not worth the effort.