The phrase "hardest difficulty" refers to the highest level of challenge in a game or task. It means that the task or game is designed to be extremely difficult for the players or participants. Full definition
The game is already fairly easy even on harder difficulty settings and adding the infamous figures into the mix only makes things easier. (apgnation.com)
Doing so is a massive struggle, especially on harder difficulty settings. (gamerankings.com)
Its premise was simple, but on harder difficulty levels, and in versus mode it's complex, challenging and competitive. (bit-tech.net)