-( Story related, cannot be missed )-
Doris Fletcher, dwelling within the Grand Guignol Theater in the center of London, has invoked the plague upon West End. Therefore, as the elected savior, you must put an end to her. She is far different from any other enemy you've faced, which you will find out after you've dealt enough damage. So, a few pointers:
1) To start, she will simply do a one-two-three attack every now and then to close the gap between you and her. You can strafe and dodge to evade these attacks and then deal some damage to her, perhaps stunning her in the process.
2) She has the ability to call in Ichor enemies (the females that spew the pollen-like substance that slowly drains Reid's health). Thankfully, they are only level 1, so if you want to spare some blood for their sake, use a Blood Spear or cleave at them after evading them to get rid of them. Just be sure to dodge away afterwards, as they will explode.
3) Doing a decent amount of damage, she'll arm herself with a shadowy appendage. This makes close combat difficult, as it whips about her when she withdraws it and it is hard to evade without seeing.
4) To avoid the shadowy appendage phase in general, deal enough damage early on and she'll stand in place, talking to you and calling you the Kinslayer. This is your immediate cue to get as far from her as you can, as she is fixing to explode ichor around her and change her outfit. You'll see that the shadowy appendage takes a more physical form, making it easier and harder to evade at the same time.
5) Now that she has revealed her secret weapon, she will use it more often in her combo attacks. She can whip it forward like the Blood Spears; whip it around and retract it; and she can call more than one Ichor at a time. Once you've revealed this form, it's simply an act of dodge and counterattack before she dies, as she won't have any newer mechanics after that. But beware that the Ichor can still spit at you from afar, spew around themselves, and explode in a hazardous cloud, so you must be aware of your surroundings once more.
Defeating Doris will reward you with the trophy and get you one step closer to uncovering the truth surrounding the epidemic.