itch.io

There's a handful of different tricks for this. The main way is making it so that the boss is only defeated once a certain number of predetermined successful actions are made against it (which is sort of a really simplified HP system for the boss). This isn't very useful by itself though, as saying "You successfully strike the monster and it seems a little bit more hurt," gets old pretty fast and starts to go against the narrative nature of the game. So, it's good to mix it in with one or more tricks...

• Give the boss one or more forms of descriptive defenses that must be broken before it can start to be harmed. Examples could be: layers of armor, a shield, a magical ward, a protective artifact, a unique defensive ability that must be exhausted, etc. You could also split up the enemy into multiple parts (it's giant eye, it's flailing tail, it's bloody claws, etc.) that must each be targeted and destroyed before the enemy is defeated.
• Throw more complications into the boss encounter than just the fight against the boss itself. Add in some lesser enemies, or some helpless victims that must be saved, or a fire that is quickly spreading, or an objective that must be secured quickly, etc.
• Give the boss multiple "forms". Once the boss is hit a few times, explain how it subtly or drastically changes in appearance, then give it new abilities or tactics that can surprise the players.
• Give the boss more things to do than just endlessly attack the players. Have them set up defenses, create additional complications, and empower themselves in various ways. This can extend the encounter while also keeping the players on their toes.
• This is a really effective and important one: Constantly put the players in a reactive position. Before asking a player what they do, put them in a situation of impending danger (such as a descriptive incoming attack from the boss) that they need to react to. This makes it so the players can only safely push the offense when there is an opening provided to them, by you, in the narrative.

I find using one or more of these can really help to extend the length of a boss encounter without it devolving into repetitive and boring back-and-forth attacks that that only drag out the session. Keep the narrative as the focus and you can have a lot of fun throwing out unexpected challenges and revelations to the players as the encounter unfolds.