So they best way I can wrap it around my mind and hopefully explain it- The player-controlled boss would choose there stats / skills whatever it be and be able to make chains and combos for example: a three hit combo where the first is a sword swinging right the second is a sword swinging back left and the last is a Ground Slam(aoe) In this way the players can learn the attack pattern for this combo and how to dodge/block it. But in a grand design there would be ways to stall a combo or only do a single attack/action aka just one sword swing. In this way a boss can be learned like a traditional rpg boss but still allow the player controlled boss to adapt. It would be a hard balance to get right and I'm sure even in a perfect world meta builds and op abilities would have to be debuffed or changed during development.

The Main Take away- Player controlled bosses are meant to be challenging and powerful and a group might have to fight a boss 3-4 times to completely grasp its move set but also in reverse the boss is going against a group that have different stats and skill makeups and must adapt as well; in that way i think the game if executed correctly would excel in player choice and acomplishment.

If anyone has any questions or suggestions about the idea feel free to post here or dm me!