Now THIS is a challenging achievement!
First up, learn the controls of the game (i.e. learn which button is to use the Wrangle Pack, I don't remember, I deleted this game when I beat it...)
Start up a level, any level, multiplayer or single player, the choice is yours!
Find a single zombie (in a zombie game, this might be quite challenging).
Press, and hold, the button that allows you to use the wrangles pack (you might want to face towards the single zombie).
If you hold the button for long enough, the zombie will disappear, and you shall get a very difficult achievement that will make your Gamerscore end in a 9 for many years to come! (I mean, only 99% of people have it, what on earth did the other 1% do???)