This achievment is actually very simple. You need to kill 15 Enemies with the machete. Note that this is not the small knife.
After finishing the game go to level select and then replay Level 7. (Captured). Play this on easy.
As soon as the level starts there is a man on the wall with a Machete in his chest, go over and grab that. Kill the first infected and make your way up there ladder. From here on out the rest of the enemies will be human, meaning it only takes 1-2 swipes to kill them. Proceed through the level only using the Machete and you should get this in roughly 4-5 minutes.
Hope this helps.