After the first immunity phase, you need get her health bar as close to 50% without going past 50% as possible. Once she fully stands up, drop her to 50% which drops her to the next immunity phase. This time dump your bang stick into her crit spot and you will skip right to her last immunity phase. Finish her with the ion cannon.
Tip - It’s better to not do enough damage to get her to 50%, then dropping her below 50% to early. The latter will force multiple extra un damagable phases, while to little one only force you to do 1 laser cylinder phase.