This, this, this.

Also I often find inspiration in mechanics from MMOs, RPGs, and boss fight games like Elden Ring.

• Shifting damage resistances based on HP phases or whoever crit it last
• Phases where the boss takes no damage and the players have to do individual actions around the room before N turns or the boss heals
• Forced side battles where one or more players need to kill a mook in another room before returning to battle
• Shields that go down when players step on specific pressure plates, ensuring they're in range of my dmg
• Capture the Flag style McGuffin delivery to kill the boss
• Boss is fleeing through a dungeon at ~1 room per 3 turns and the party has to complete room objectives to reopen doors/keep up with him/keep LoS.
• Boss has resistances to all damage until hit with N conditions when those resistances drop for N turns