This is the final boss battle of the game. You bring down her health in increments of 20%. Each time you do, she will start using a different elemental attack (fire, poison, electricity). Whenever she loses 20%, she will go into a shielded mode where you can't deal any damage to her, and will spawn a bunch of easy enemies to kill.
Here is the order of her attacks if I remember correctly:
After first 20% is removed, she uses fire
After second 20%, she uses poison
After third 20% she uses electricity
After that she uses all three
Hope this helps. I used a sharpshooter and managed to pull it off with only two deaths (somehow always die to poison in this game).