"Boss monsters" refers to powerful and challenging enemies, typically found in video games or fantasy worlds, that players have to defeat in order to progress to the next level or obtain rewards. Full definition
You'll only encounter a handful of boss monsters throughout the adventure, and all of them are variations of the same two bosses. (wizarddojo.com)
This can be anything from boss monsters and hostile wildlife, cave ins, to just falling a little too far. (gamerassaultweekly.com)
This tactic is a bit more difficult with boss monsters, more so if you're stuck in the middle of a combo move, but is incredibly useful against lesser beings. (darkstation.com)