After playing with this mod for awhile I've put together the best guide I can if you're trying to kill them.

1. RETREAT IS AN OPTION. Every none serious team that encounters them tends to get rekt, even with 3 trinkets each. The stress penalty sucks but it'd preferable to losing heroes. Don't ever forget this, even if theyre in the way of a mission critical path.

2. RESERVED GRAIN. Not much to say here, make sure to have a fast corpse clearer in your squad, regular damage is unadvisable as the corpses have high health and spawn in the back. If they're able to eat a corpse they will power WAY up, this includes eachother! Because they kill themselves with a skill you can't remove the chances of them leaving corpses with crits or DOT. Upon eating corpses they gain many more attacks than the one or two they start with and even gain another turn, or even 2 to act in.

3. REMEMBERENCE PART 1. The enemies kill themselves with this skill, leaving behind a corpse for their comrades to eat. I can't say Much here aside from keeping other corpse cleaners who will hopefully act before they can eat the body, no solid way to prevent this. Once again, THE FIGHT BECOMES INFINITELY HARDER WHEN THEY EAT CORPSES, DO EVERYTHING YOU CAN TO PREVENT THIS.

3. REMEMBERENCE PART 2. Each enemy buffs the remaining enemies when they kill themselves. The front enemy adds dodge to the already fairly high dodge value these enemies have. 20 on the veteran mission I just killed them in. In truth I don't remember the other two as well as the first, one of them heals their comrades on death, the other adds some kind of buff, again I don't have everything remembered perfectly, but I'm fairly sure the second buff is as nasty as the first I mentioned

4. NO TIME TO BLEED. Another fairly simple point, they have very, very high bleed resistance, rendering bleeding classes almost completely useless, on the flip side they have fairly low blight resistance so if you want DOT use that, however...

5. PROT-BE-GONE. ...They have zero prot, and do not gain any throughout the fight. Which means in my humble opinion nuking them with straight damage is the best course of action.

6. BIOMES-O-PLENTY. This boss drops different fabric depending on where you beat them, i haven't encountered them in the farmstead and have yet to enter the courtyard but from what I've seen the fabric you get from the Warren's is absurdly superior to the other options, the effects aren't combat oriented but are incredibly valuable as a utility.

7. INVALUABLE FABRIC. Aside from their trinket effects every fabric drop this boss produces is worth FIFTTY FIVE THOUSAND GOLD. 55,000. This is not a joke. Not an exaggeration. It's invaluable if you have mods that have super expensive things. There may also be a certain coat made from these fabrics that this boss can drop that is not only far more valuable but also many times more powerful as a trinket than all the individual fabrics combined.

8. MISC TIPS. I've told you all I can about the main boss, so here's a couple thoughts.
While they're vulnerable to blight they have lots of health, lots. However, after they eat corpses they gain extra actions, doubling, or tripling potential DOT damage they can take in one turn. In the end what tactics you use should depend on the party and the state of said party. I'd still nuke them with straight damage though.
The starting attack they have before they eat is very weak damage wise but adds a significant amount of stress and horror on top of that, bring a stress healer if you can afford to.
Sometimes this boss attacks before killing themselves when brought to 0 by DOT, this situation hasn't happened enough for me to say its consistent but you should keep this in mind.

AND... That's it. I've prepared you about as much as I think I can here, they show up fairly often so if its too often try the alternate mod that should show up when you search canival or something similar, I'm doing this on my phone so I can't check without losing everything. Best of luck, heir.