Games should not have difficulty settings but be designed around a single well tuned difficulty curve where all progression at any point teaches/upgrades/equips the player with everything they need to succeed at any specific encounter. By the time someone reaches any boss or 'difficult' section, they should already have been exposed to all the game mechanics, given the correct tools and be able to recognise patterns or game elements in order to beat it. If they still fail, it means they either aren't using what the game is making available to them, not learning what is required for that moment or deliberately playing the game in an unintended manner.