Overseer

Boss of the true ending path. You will face off with her, at the end of the final true end portal. She cannot be recruited by any non cheating means.

Stat wise, the Overseer is the strongest boss monster, cap breaking genes, high growth across the board, hitting roughly 100 damage per attack and blocking massive amounts of damage with her 100 armor. Similar to the Horror boss, the Overseer requires overwhelming offensive power to deal with. Overseers counter item is a story based craft able armor removing spear. Other than that, she is humanoid, so she is susceptible to lust based debuffs. While breaking past the armor can be a challenge, there are few other ways to deal with her. With sufficiently high enough hp, roughly in the 300s, and the skill FullHeal, you can simply chip away at her armor, as each hit lowers it by 1. Alternatively, you can debuff her to hell, with a combination of statuses, like cursed, soaked, weakened and increasing her lust above 50%. If the Overseer is weak enough to not wipe your team, you can dogpile on her and quickly remove her armor, the assassin unit is especially good at this. A few alternative armor ignoring methods would be stacking burn, or using the skill Consume, to one shot her.

One especially reliable method is to bring a couple of hard-hitting units with the PiercingClaws trait (such as Mirrea). Since this lets them completely ignore the Overseer's armor, the fight then becomes a much more straightforward damage race. If possible, make sure that your piercers have ManaWorms in their consumable slots so that they can alpha-strike the Overseer in one or two turns. As this strategy does not require you to craft the armor-stripping spear, you can instead use those materials to craft the item which negates Horror's powerful passive effects, making the whole encounter dramatically easier.