I think i some good ideas how to adapt the bosses to the player strength.
1. Status effect damage that increases with the max hp of the player.
2. Status effects that keep the from attacking( (Freeze, Stun, Silence) or decrease the players hit rate (Blinding).
3. Higher Dodge Chance, Self Healing , but a decrease in Bosses HP
4.Bosses with the ability to parry/ counter attacks and throw a portion of the damage back at the player.
5. The chance to resist spells like Pharao Mommy.
Something that is not directly to combat difficulty, but still could be cool is special interactions with boss, like the special attacks in the Kallista Duel.
Or a kind of wild idea is temptation interaction where the boss, tries to seduce the player into giving up the fight for pleasure. Resulting in Femdom/Domination scene (cowgirl, etc.) and the player losing fight.
I think a similar system has already been in Yorna.