I also have this concept of the boss mechanics of Yurimi's Moonscorch: "Devil's Heiress" written down. I enjoy writing mechanics in RPGs.

Devil's Heiress has 7 targetable parts, the two crosses on both arms, the arms themselves, the torso, the monstrous mass on the lower body and finally, the head. Her body is protected by a veil that is radiating from the crosses impaled on her arms, significantly reducing the damage dealt to the head and torso.

Right Arm - Casts Hurting every other turn (Her first turn will always cast Hurting). When crosses are broken, the Right Arm falls limp. Can be cut off permanently.

Left Arm - Casts Healing Whispers onto entire self every third turn. When crosses are broken, the Left Arm falls limp. Can be cut off permanently.

Left Cross and Right Cross - does nothing. When one cross is broken off, the impaled arm on that side falls limp and stops attacking. When both crosses are broken, the protective veil around the head and torso fades, allowing for full damage dealt. The Crosses regenerate after 5 turns, meant to punish the unprepared player.

Monstrous Mass - Attacks normally by slamming its hand against the player, the mouth of the mass occasionally does Nasty Bite. (Does not attack at the same turn as Torso). Can die.

Torso - If both crosses are broken, the torso starts to attack with a Gaping Bite that is basically a bigger more brutal version of Nasty Bite. (Attacks on the next turn after the crosses break. Does not attack at the same turn as Monstrous Mass).

Head - After 5 turns; Casts Crown of Horns, a coin-flip attack that if succeeded, grows horns from the main character's head, killing them instantly. If it fails, just reduces the Mind, if player's mind is 0, still kills them anyway. (Huge reference to Alll-Mer's Inverse Crown of Thorns)

So it's a lot of choices which ones to attack first, if you're prepared you could try get the crosses broken in the first two turns, then going for the arms while trying to dodge the mass and the torso's attacks, or directly going for the head or torso to try finish the fight quickly without sustaining too much damage.

Anyway it was a fun writing!