Tiny Tina's Wonderlands

In the beginning of the game you'll learn how to melee in the Overworld to get rid of small branches in your way, and you'll also learn about how walking through tall grass leads to random encounters. For this achievement, you need to walk in tall grass in the Overworld until an enemy pops out and starts to come after you, then simply melee the enemy before it hits you and starts the combat encounter. It kills the enemy entirely (rather than actually stunning it) but it still unlocks this achievement.