Before you start equip an insta kill consumable if you have any, It'll make this a lot easier. If you don't have any equip the Resourceful mod so you spawn more power ups.
Build the Ripsaw, upgrade it to where you can shoot sawblades.
Train zombies so you have at least 13 chasing you. I found it best to train them in the u-boat room and waited for them to stop spawning.
Now you want to get an insta kill, if you have the consumable now it's time to use it. Try to get them all in a straight line before you shoot.
If you got it right the achievement should pop about 3-4 seconds after. It took me 4 tries to get the shot right.