Seems bugged that this boss only takes about a half to a full seconds worth of damage over 5 minutes. I know because I just did this on T1.
Plz fix.
While waiting for the fix, here’s how you can kill her.
You need to stop hitting her on each 1/3 of her life, to let her invoke the adds. If you continue to hit her, you will skip the first invokes of adds, then she will be bugged. So lower her to 2/3 of her life, then stop and wait for the adds, kill the adds, then lower her life to 1/3, Stop again wait for the invoke of adds, then kill her. Then P2 is no bugged.