Dragon's Dogma: Dark Arisen

The easiest way to unlock this achievement is by killing the hydra that challenges you via the notice board in the Gran Soren Pawn Guild during the post-game. If the quest isn't there (it's called "A Challenge"), you may need to complete another quest first and then it will be available. Alternatively, you can fight one in Bitterblack Isle's Rotunda of Dread at any time (though the enemies spawn randomly so you may need to reset it multiple times until you get it). As for the archydra, that can be found in the Chamber of Hesitation in the Everfall at any time in the post-game (after beating the dragon).

The best way to kill the hydras is to have large AOE spells from a powerful mage, or have a fighter climb their necks and cut their heads off. They don't have too many dangerous attacks though.