The Minotaur can be found in Messania, in the South East corner of the map. If you roam a bit around the mainland you will find the ruins of Knossos. There you will find a young boy that will give you a questline. At the end of this questline he opens the way to the labyrinth hidden under the palace. Follow the golden thread on the floor and eventually you will come to an open space where the minotaur is waiting for you.
The minotaur fight is fairly simple and straightforward. The beast will throw rocks at you, then charge. Dodge out of the way and pepper him with arrows, while going for melee attacks when you pull off a perfect dodge to slow time. He hits like a truck, so be careful not to be to overconfident.
At some point during the fight, when he's down to about 1/3 of his health, there's a small cutscene of him smashing the arena a bit. The playing field is now significantly smaller, and he's much more aggressive for the remainder of the fight.
Just keep dodging and shooting and eventually you'll bring the beast down. Loot the corpse, watch the cutscene. Afterwards there's a small chat with the boy again before the achievement unlocks when the quest is finished.