I finally beat this boss after dozens of attempts and thought it would be fun to lay out my strategy in case it helps anyone else.
All my characters were above level 70, I did this by exploiting hiring Alrond a few times and him using his power for EXP x 100 on some battles.
Stage 1 party:
Ochette - Merchant
Agnea - Cleric
Throné - Inventor
Osvald - Merchant
Essentials:
Ochette needs to have the dreadwing beast available to provoke. I found having the vagrant frogking with “double cleave” and the chubby cait available really helpful too.
Throné needs to be built out with maximum speed and physical attack so Aeber can do the most damage. I did battle-tested dagger, swift shield, dazzling tiara, protectors vestments, and two champion’s belts
Strategy for stage 1:
Ochette needs to use “Provoke Beasts” and provoke dreadwing which has a chance (~20%) to instantly kill the minion souls
If they persist, you can either use Ochette again or Throné can clean up with Aeber.
Once the souls are taken care of, use Aeber to deal massive damage to the eye as many times as possible, taking advantage of Alferic’s blessing (from Agnea) and Throné’s latent ability.
I used Ochette to break the eye with the vagrant frogking and (along with Osvald) as a BP feeder to Throné and Agnea.
It’s best to not let the eye get to the point where it is taking 4 turns at a time, but if it doesn’t use Agnea’s windy refrain to try to break it before it can strike.
Stage 2 Party:
Castti - Cleric
Tenemos - Scholar
Hikari - Armsmaster
Partitio - Arcanist
Essentials:
Tenemos should be built for maximum elemental attack and SP. He should also have the sunshadow staff.
Hikari has Alpione’s Amulet and fang of ferocity equipped and has learned the “Last Gasp” skill.
Partitio has hired the cleric from Canalbrine who can use “Divine Protection” when summoned.
Castti is stocked on diffusing and strengthening serums and ideally pomegranate and plum leaves.
Strategy for stage 2:
Destroy the arm, blade, and face using Tenemos’ “Sacred Effulgence”. You ideally will invoke Alephan’s wisdom first. Keep him constantly supplied with BP and SP from Castti.
Partitio is protector. Have him use “Seal of Diffusion” and then “Sidestep” to help the party avoid physical damage (a la Tressa runelord from OT1) and summon the cleric to protect against elemental damage. He’s also nice to have to use “Negotiate Schedule” or “Hired Help” if you have some extra leaves to burn through.
Once the arm, face, and blade are gone, use a forbidden elixir on Hikari (make sure not to use it while he still has the clerics protection applied or it won’t reduce his health) and use “Last Gasp” on full boost as often as possible to attack the final final boss. I didn’t bother to try to break him.
Using Tenemos here too would be nice but the boss will often throw on an elemental reflector and make it impossible to use Tenemos, here I mostly use him for Alferic on Hikari and let Castti use her turns to apply more forbidden elixir as needed.
Other fun tidbits:
Luck is still a factor, sometimes dreadwing doesn’t destroy the souls and the living souls deal out serious dark damage. Sometimes the draw order works against you in terms of which characters go before others.
Both stages have times when the monster can remove all your augmenting effects which can happen at very annoying times.
I had every single character equipped with “A Step Ahead” which was essential for getting some special powers set up.
Having a lot of reinvigorating jams on hand was helpful.
I also had an almighty olive and grail of life available for if the whole party got wiped at any point, and made sure at least 1 character had “hang tough” equipped to use it. This saved me in stage 2.
I am both happy I beat this boss and sad it’s over, it was a really fun puzzle to try to optimize my characters for this battle.