When I first encountered the sewer beast I got extreamly fustrated and threw a open soda bottle at my computer monitor, and then I took a minute to find the most pain stakeningly grindiest way to kill him so that is exactly what I did before you start you will need a few things.

What you need before the fight:

Long Surgical Knife with atleast +5 blood absorption.
Claw ability.
Perferable Cuagulate ability.
What to Do During Fight
So what you do is simple you wait for him to attack, dodge, attack with surgical knife make sure you hit two times, then hit four times then use claw once.

Repeat that process and when ever you are low on health those two times u saved, are for Apothcary to heal your self this will not work the first try you will have to do this over and over until you get it right, BUT this is the best method I had found of deafeating this darn sewer beast.