Finding the perfect balance of challenge can make or break a game. If a game is too easy, players will find it boring and breeze right through it. If a game is too hard, players will become fed up and walk away from it. Challenge can be anything from damage received from enemies, complexities of puzzles, reflexes needed for events, and so on. While difficulty is subjective, poor game design is not.
Source：https://www.cbr.com/most-annoying-boss-fights/
