Star Wars KotOR did this really well with the final boss. He heals himself but it makes sense why he can and you can prevent him from doing it if you know how.

Boss/enemy healing isn’t bad game design. There are a lot of very valid reasons that lead to more interesting interactions.

• A boss with slow health regen might require players to try to burst the boss down through weakpoints or debuffs

• A boss entering a second phase and getting their health back creates a cool “oh shit” moment and really emphasizes their power

• A boss that has an interuptable or preventable heal adds an interesting challenge and requires players to do more than just DPSing them down