Dungeons and Dragons 5th edition actually has an interesting way to handle bosses.
Simply speaking bosses get their main turn, then they get 1-5 "Legendary Actions" which are a list of moves they can perform between other PCs turns with different Legendary Actions costs.
Bosses also are less immune to everything in 5e and instead have "legendary resistance" which gives them a few free saves against enemies.
Say what you will about Dungeons and dragons 5th edition... It probably has the best system for bosses I've seen in pen and paper and could easily be examined for videogames as well.
The trick in turn based games is not to let bosses get overwhelmingly disadvantaged in the action economy. By giving them more actions, it means you don't have to make them unfair.
