I hope this isn't too late, but if you want to really want to build up to the battle, have the boss harass them throughout the session leading up to the final encounter. Some good advice for running that can be found in the D&D Beyond article "How to Play an Oni like a Living Nightmare." Playing that helps build the narrative of the boss, gets the party to really fear the enemy, as well as a hit and run tactic when building up to the boss allows the final fight to be even more frightening due to the resources they've used from the early, split second engagements no longer accessible to them.