Wanted: Dead

This Achievement can be easily obtained on the very first mission Dauer HQ. In the huge courtyard after going down the steps is a chainsaw highlighted in yellow. Press down on the right stick to pick it up and it will swap out whatever secondary firearm you're carrying. As soon as you enter the courtyard a large number of guards will charge you, press X to melee like you would with the katana and you'll get an instakill with the chainsaw. It even works on the ninja that appears before you enter the lobby in front of you!