Contrary to the other solutions the quickest place to get this is in Gor'Castle Parks entrance (Credit to Das Chase on this). Take the boat ride, and skip the cut scene by pressing Y. Go to the right until enemies spawn, hopefully you get one or two machete guys, if you only get 1 machete go back left and then return right and magically enemies will respawn for you. Once you get two machetes proceed with the limb hacking (focus on the fat kids, machete, and chainsaw dudes) until the achievement pops. I cannot verify if this is progressive or if it has to be done in one go, by the way mine popped I would assume it is progressive but don't quote me on that.
If you have put some wear and tear on your machetes you can always go left one screen and then return, it heals your weapons so you don't have to spawn machete dudes again.
IMPORTANT NOTE: This WOULD NOT unlock for me as the guest in a coop game, we tried for about an hour and I quit in frustration. Once I vented long enough I played to that point in single player and it popped in my first attempt. Good luck.