The Doris Boss Fight in Vampyr takes place in the theater during chapter 4. Doris Fletcher is a very powerful opponent, especially if you are playing to get “Not Even Once” and are on a low level.
Doris’ Level is 30 and if your character is around level 19 like mine was here, you are going to have little room for error. Watch out for her sword swings that often come in waves of 3 and later on her mutating arm that can hit from a distance. Always try to read here and only come close to deliver quick hits.
The Ultimate Blood Cauldron is the best weapon against Doris. This ultimate deals a massive amount of damage and will most likely end the fight on the second use. Upgrade it as far as you can and Doris will be down after you learn her routine.