It's always easy to beat Maggie/Ada/Sarah (the first time she appears) with not bad luck and good habits (use bandage in time, not always dodging or bandaging), so let's just talk about Ariana and Sarah (the second time we meet her)
Ariana is the boss that can shoot 4 bullets in a round. Let's dodge for the first two rounds because we'll take 60 damages if we don't (bandage can heal 50 per round)! She might only shoot twice at the third round so let's shoot back (also to avoid keep dodging for too long). Then we dodge twice, use bandage and shoot her while she's reloading. It seems that it's 100% when we use single shoot to her but not for double taps. She'll fire again when she finishes reloading for only 6 bullets (half of the full status of her), so let's just dodge twice (she'll shoot 4+2 and then reload again). thus, we can keep our HP at 100 always (but only bandage and shoot when she's reloading later). In this way, it'll be easy for us to go to next battle with 100HP and 5 bullets which still make nonsence.
After defeating Ariana, Sarah will come to fight us. This time the system will say that we have no more bullets and in the old version, Bo Wei said that it's designed to be impossible to beat her this time. I don't know if it's changed in this version but it seems not. My best record is to shoot her successfully with a double tap so we lose with Sarah 150HP left. Hope that there'll be more people trying it and eventually find out a way of defeating her.
BTW, what's the difference of this version to the old one? I didn't feel anything updated I think.