The Last Stand: Aftermath (UP)

Once you get to the final region called "Hell" you will encounter a few new enemy types. One of these is called an armored charger. He is identical to the normal charger except now he has armor on him which makes most firearms useless against him until you can melee it off. Make sure to avoid his sprinting charge as this could be fatal in most cases. Get up close and melee him until his armor is off and you can hear your weapon hitting flesh. As soon as this happens sprint away and then unload your firearm on him. He will eventually succumb to his wounds and die which will unlock you this achievement.

Cheers hope this helps!