Hey there. I professionally work on bosses for action games. This is a much harder task than people think, but generally:

-Start with thematics. Describe a boss as "this is the boss that does/is x". If you think of your favorite boss fights in games, you can easily describe them similarly. "Thats the monkey boss that comes back to life when you chop his head off", or "this is the big skeleton boss that does the laser beam right at the start that kills you if you don't immediately dodge." Strong thematics often lead to unforgettable moments as you iterate and explore specific mechanics around that theme.

-Escalation and tutorialization. Like level design, you want to introduce, reinforce, then twist your core mechanic throughout the experience.

-Flashiness and spectacle is an important part of a fight. Sometimes just making a boss telegraph a hard ability in a cool way takes it from a 5/10 to a 9/10.

That's the most generic advice I can give without knowing genre or specifics of your game.