Help me with the final boss fight

[agile peak]

My party is reaching the end of their campaign where they will enter a portal to fight this giant demon. The thing is... I don't want the fight to just be a slap-fest. Any ideas on how to make the fight more like a puzzle? or something unique like that? 🙂

[round vortex]

What level are they?
What kind of demon is it? Even if it's not a specific kind, they usually have some sort of main sin attached
And how confident are you in improvisation?

But regardless of these, if you want to have something other than just fighting then you can perhaps have them be in the middle of performing some sort of ritual that, if successful, will make the demon completely overpowered (and the party would know at least that the ritual is dangerous and should be stopped at all costs)
And while the ritual is being performed the main demon would be invulnerable, but also limited in what they can do.
Once the party removes the adds that are performing the ritual, phase 2 begins where the main demon uses their entire arsenal.

Remember to include some sort of non-opportunity attack movement / teleportation so that the boss isn't stuck in one place all the time.
Good options of forced movement often work as well, though one needs to watch out for barbarians with their advantage on both str and dex saves as well as at least passable bonuses

[void bison]

Add secondary objectives that would support the main objective of killing the giant demon. You could take a page from Demon's Souls' Dragon God fight, and have a couple magical artillery, littered around the fringes of the arena, that would incapacitate the boss in some way.

Some kind of interactable that's out on the field, or weakness that they players would realize they could capitalize on

Something that puts certain enemies leagues above others is adaptability. If they can shift their fighting style to be better at fighting against a certain player or two, but then lose resistance to the other players' attacks to compensate.

I can imagine the demon fortifying their physical defenses, heightening their physical attacks, but then becoming less resistant to magical attacks. Or strengthening its fire attacks, things like fire projectiles or fire breath, and becoming more resistant to magical attacks, but losing resistance to physical attacks

[unique fiber]

3 things which make any fight more fun

interesting maps which makes creates challenge for players while benefits the bbeg

Some no save gimmick on bbeg end like automatic grapple or push or even charmed unless party closes eyes when fighting or something. Maybe seeing bbeg is lossing he picks a party member and flies higher up on the map meaning unless others get there quickly its a 1v1 possible death situation for that particular party member.

Make something else higher priority than simply beating the bbeg. Maybe bbeg has a hostage who must be kept safe during the fight, maybe there is some magic item which has demon's heart in it. Maybe the only sword which can kill the demon is his own sword.

Such situation gives more rp options than straight up fighting or at least some advantage and gives feeling of accomplishment to players.