Monster Queen (IIRC, she's called the Frost Queen or something?)
She's the antagonist of the Intelligent Monster's route, which is insanely stupid to access.
To access it:
-You must have Julia
-Julia must have Detect IIRC (1200 Support proficiency - yes, you read that right. Have fun...)
-Julia must be in her Normal Personality
-Julia must be part of your active party (meaning you bring her to the dungeon).
-IIRC, you must have obtained the Berserk spell for Ashley (level her a LOT - try being level 25-30 at minimum).
Lose 2 battles against any 2 random mobs that don't give a unique ending (for example, Baby Slimes).
Lose 2 battles against 2 Monster Girls (Flower Girls, Slime Girls etc).
Enter the dungeon with a party consisting of the Protagonist, Julia and a third member who is not offensive (so you can't bring Val, Sarah or Derrick for example - Cass or Molly is fair game to bring).
Wait around and see a scene in which Julia notices monsters following you. Choose either option.
I forgot how long it's after that, but I think it's 3 more scenes (I could be wrong on this), a boss fight will occur... during which you only have access to Ashley. Who you can't control. Because she's permanently in her Berserk state. Which is why I told you to level her a ton.
There's four possible outcomes:
-You lose: game over.
-You lose: Julia has her gun from the Julia's Arsenal route. Julia shoots the Frost Queen and saves the day. It does not matter if Julia got help from Brittany or not.
-You win: Frost Queen gets her neck broken by Ashley (automatic if you win).
-You win: Frost Queen gets enslaved by you (there's like 3 lines total where you can cast a Chakram on the Frost Queen after realizing she's pretending to be dead). This unlocks her in the Monster Ranch for your amusement.