Boss
From Styleguide
boss
A notable enemy, usually one possessing much greater power than other foes in the game. A boss is typically found at the end of a game level.
Capitalize specific boss names.
A major enemy that comes before the end of a level is sometimes referred to as a mini-boss. The last boss in a game is the final boss.
Example: Metroid's Mother Brain remains one the most unforgettable final bosses in gaming history.
Wrong: boss character, end boss, miniboss, sub-boss.
