This first becomes available once you reach the Lost City section late into the game.
(Using an xbox 360 / Xbox one controller) hit B to dodge as they attack and the game will go into slow motion. Press Y when the button appears on screen to do a counter. It should take about 3 or 4 times and you can then finish them with Y when they are on the ground.
I don't recommend doing this during the first section when you encounter Deathless since there are so many archers. Wait until you reach the main city segment (with the trebuchets etc)