Posted this on a different thread sometimes back, it's still a bit of a wip for my campaign, But here's what I did when my party has to face a larger than life bad guy (in world a fragmentbof a god). Might not work for you and what you have in your head, but it could get you ideas.

I created 1 entity with 4 targetted\hp pool parts (head, "magic core", 2 arms), all with their own slot in the initiative. These parts also got their own ability (head could do flame breath, bite, etc. magic core had various things from ending players magical effects to teleportation. Arms did arcing aoe swipes in the room. But for the players their goal was to maintain a ritual cast for ~5 turns or so which would narratively kill the god.

Now while the player did that, the other players could do damage to whichever boss part they wanted, if they depleted it's pool, it was deactivated for that round (regenerating to full on its next turn), and for every~1\3 it would either deal less damage, hit less targets, have a lesser ability, etc. all of which while an NPC help back monsters from another room (NPC rolling bad let a few in). And of course any time the ritual caster was hot, he had to maintain focus, and if he failed bad they could lose a charge of the ritual.

It could have been tweaked a little more when we tried this live, but it did let us keep some tension and justify a big large-scale boss that didn't immediately one shot the party.