This achievement is very simple when you know what your looking for.
Ok during the majority of Episodes there are Gas/Propane Tanks scattered throughout the Campaign.
If you shoot at one next to a single Zombie this will be 1 of 10 and 10% on your progress bar.
Point to not when your sight cursor dot/reticule changes from white to red over a tank your golden to pull the trigger.
Image below of Gas Tanks:
If you shoot a Tank with a horde of Zombies this will count to a multitude of kills.
Walk/run through your natural campaign shooting at these with zombies next them.
In the image below the Tank in the distance was a great one for me, as a bulk of zombies gathered by, this was at the section when you protect the crazy train driver in the Episode 1.
As soon as you kill your tenth zombie, pop achievement unlocked!