If you want cinematic boss battles then the first thing that you may be forgetting is to speak with your players beforehand. The only real way that you’re going to be able to pull off a boss battle that feels cinematic (which I’m really getting that you’re wanting it to be extremely climactic and inventive rather than feel like a ‘scene’) then you might want to entertain the idea of taking control of player actions for short scenes during the fight intermittently.
Think about your favorite video game monsters for example… Whenever fighting with some of them they might bash through walls and enter into a new chamber filled with horrors and traps that they’re hiding behind because they’re realizing how much more powerful their opponent, the protagonists, are in the first place.
I would say for epic monsters that one of the best games for you to look up source material from is probably “Shadow of the Colossus”. There are NO mini monsters in that game and that means that the monster bosses have to be that much more epic. There is nothing wrong with planning no monster minions at all and throwing only random wandering monsters at your players if they’re seeming bored instead… while description leading UP to the monster might give more of the heightened effect you’re wanting if you cannot really think of what must be done to create your boss fight to be as epic as you’re hoping. Like all the designs of the tomb that the party is venturing into, including writings and runs from thousands of years ago that tell the tale of why it is that wrath came into being in the first place… The anger and hatred of the first murdered soul that went unanswered due to its inability to seek justice on the other side of the veil created its spirit, and hatred in the form of bigotry formed its body in the hearts of men…. And so forth.
I’m kinda rambling now, so I’m going to stop.
I wish you the best of luck with your epic designs.
"I am not young enough to know everything." ~Oscar Wilde