To unlock this achievement, you need to replay any level that you have previously completed, but you need to finish it with a new gun. There is a chainsaw on the first level that you cannot get until you have acquired the Jump pad later on, so you need to leave this weapon at first. When you have gotten the Jump Pad, back out to the main menu and begin the campaign again. This time, using the jump pad, obtain the chainsaw that you previously left behind and continue to complete the level.
The achievement will unlock upon completing the level.