In this video guide I will show you how to kill and enemy by knifing them with a gun blade.You can do this at any time you find a weapon with a blade on it. If you can't tell for sure the weapon has one you can click RS to be able to put it in view mode. From there is will show it zoomed in. All you need to do it weaken an enemy with your gun and move in to finish the kill with the melee. The achievement will pop if the melee got the finishing blow.