Sea Salt

I started the sewers with the guy who starts with the cultists. By the time I got to the boss area I had 5 cultists, 2 shaman and a ton of fishmen. On the immediate left and right their are summon spots where I did one as another set of cultists and the other a set of fishmen.

The first phase of the boss I dodged the first attack then quickly ran back to him and attacked with everyone and he died quickly.

A mini cutscene happens and you go into the 2nd phase. Since I was swarming him I was already at him and I just kept holding RT during the cutscene. As soon as it ended I ended up killing him in around 2-3 seconds with most of my cultists that I started with.

He drops alot of money when you damage him to a certain point so if you are having trouble in the first section and still have a decent amount of cultists left you can try hit and run tactics to use that money to summon more cultists.

Story related, can't be missed

This is the penultimate boss. It doesn't matter which route you take after Hogstead to progress through Nova, you will fight him.