It will kill you by driving your Heroes insane & waiting for them to kill themselves. The Crew heals itself, is hard to kill, and deals large amounts of stress. The Drowned Crew - not very difficult, but it's a stress fight. The pounder itself has incredibly high prot & takes ages to kill. Spawns a fusilier every turn which you have to kill or incapacitate somehow by the end o/t round, otherwise the Pounder has a chance to deal enourmous damage to your entire party.

Jester's Harvest & Plague Doctor's Plague Grenade are also highly recommended. Since it has only 1 health bar, the bleed is 4x as effective. Has one big weakness: Houndmaster's Hound's Harry ability, which bleeds all 4 enemy positions, hits all 4 pieces of the flesh. Sometimes it's a huge damage dealer, other times it heals itself, some pieces of it stun you, etc. The Flesh consists of 4 connected pieces that act independantly but have one health bar, & constantly change shape, function, resistances, etc. The Formless Flesh - probably the most difficult fight, next to the Hag. Also has an attack that does enormous damage, so an occultist, or even two, w/ weakening curse, is recommended to lower his damage.

and not Lepers, Crusaders & Bounty hunters. The Prophet hangs around the 4th rank the entire time, So bring heroes that can reach there, like Hellion, Grave Robber, etc. Bring 4 holy waters & use them on your Heroes just before the fight, as this increases the chance of resisting the siren's call. Can be devastating if you don't pay attention to your party compisition. The Siren - can 'tempt' your heroes to the enemy's side for a few turns. You should decide for yourself which boss to do next, but here's a very basic summary o/t remaining bosses: (obviously, don't read if you don't want spoilers, but i'm assuming you do)

Darkest dungeon the necromancer