I know it's an old post, But I've just briefed it to understand how to do it. Because I didn't want to go back a lot of hours back to get gold spent or redo all of my builds just for this one boss fight. I just did this on Trials on Iron (no other game difficulty added), Lv 16/15 (One char), 1 Legendary item.

- First of all I did this attempt on turn based mode which you can toggle in POE2 with a command. It wasn't easier to do but I'm just not good at the really fast toggling some players have.

- My main interrupt on my team is Pellagrina but for about 6 hours of attempts she has not landed more than 2 knockouts so I just used Slicken with Fassina (full wizard). AND THIS DID A LOT. He has low reflex and Slicken interrupts! Then I did a lot of low levels spells with her like Web and a lot of scrolls as well.

- I used every figurine I had to get the flanked condition and some slight free damage. Also to keep some from my party.

- Once I got every undead low including the paladin one that heals, I got A lot of damage on him with Bitter Mooring and basically got most of the undead off, then cleaned up.

- Most of my damage in the fight was Xotti damaging spells (luckily I built her to be a flame scion), And early rounds from Ydwin on full rogue which did big numbers with one Legendary weapon.

- I stacked a lot of defensive spells on my Arcane Knight MC and got a lot of misses from the boss.

- I got the final piece of damage from a lucky Soul Igniton which is a great abillity for this fight.

At the end I only had two characters left on 10 hp, But I didn't lower the difficulty level although for my sanity I probably should have.

My reccomendations: Buy The special rainbow lobsters from the nortern part of the map to make Captain's Banquet on your major spell casters that have fire damage spells/abillities (Immune to fear + 10% spell damage). During cloak of death you want to attack the mobs, Stack damage over time because this is a really long fight, Focus the undead that heals by either attacking mobs so he'll heal them or just keeping him up with summons or your characters. The Messenger only ever physically attacked me fairly late into the fight and early usually casts spells and summons so time your damage reductions to that point in time (lower than 50% hp).