Char personal story - In the ruins
I ran into some difficulty with this quest. I am not the best player, but I hold my own and couldn’t beat him when I ran into him. It can be the set of skills you are using. I had a heavy control build (m/s & hammer) which is fine for most things but not a regenerating warrior partially immune to control abilities.
You might find it easier to use a rifle and kite him to death. Rytlock will help you kill him, but they key is to keep him away from you. I suggest not running into the main sewer as that made him invulnerable when I fought him with my control build. not sure if this is intended or a bug.