7. Alma (Ninja Gaiden)
https://www.youtube.com/watch?v=VGNhV1MiUNA Before Dark Souls was tormenting players with the Bed of Chaos or Lord Gwyn, there was another action series with a sword-swinging hero that relied on brutal difficulty; Ninja Gaiden, developed by the appropriately named Team Ninja. The first Ninja Gaiden game was praised for its firm but fair stance on difficulty, only punishing the player if he or she made a legitimate mistake and never before. And you were bound to make a mistake when fighting this boss, the fiend known as Alma. The challenge level of Alma is through the roof, as her attacks come quick and without hesitation. She unleashes a flurry of different attacks at random intervals, never quite allowing you to lock down a pattern or memorize her moves. And even though she does have a set number of moves to use, Alma switches through them so sporadically, that trying to predict what she'll do next is impossible. Going back to the Alma battle is an exercise in frustration, but she allows you to hone your skill with the game better than anyone else and become a true master ninja.