Can be obtained when fighting the very first enemy in the game, a skeleton archer, during the tutorial level. (They also appear often throughout the first world, and less so as you progress beyond it).
Stand with a good bit of distance between yourself and the skeleton, making sure you're both on the same level, and hold down the
button to constantly attack. The skeleton will fire arrows every so often - if one collides with your barrage of attacks, which it will, you'll unlock the achievement.