The Axeman Achievement in Medal of Honor: Warfighter
Killed 25 enemies with melee during the Campaign
How to unlock The Axeman
To melee an enemy, click while not aimed down your sights. It is best to kill enemies from behind as it is risky otherwise, especially on higher difficulties. You need to kill 25 total enemies using melee to unlock this achievement. Good missions to boost this if you are having trouble are "Changing Tides" and "Bump in the Night" (Pistol Section).