Add in an extra mechanic aside from “deal damage to boss and don’t get hit” Changing terrain (platforms moving up and down), hazards (pits of lava or acid in what would normally be ideal positions), or a “switch” that can disable either a devastating ability or stun the boss when a certain somewhat difficult and specific action is done It’s also important to play to your player’s strengths If someone specializes in fire damage, maybe have the “switch” give the boss vulnerability to that
And if a player is resistant to acid damage, maybe scatter some pits of acid around the battlefield Your players are going to have a lot of abilities and giving them the opportunity to use them is important And once the players reach a damage threshold (50% of their maximum HP for example) end the fight at the most narratively satisfying moment. It keeps the flow of the narrative without dragging out the fight