Tip: Dead bodies also count towards the achievemnt, thus it is not necessary to kill 10 zombies at once. However, dead bodies respawn after a short while.
In the Pinai Ferry Station, go to the area behind the burning cars, there is enough zombies to agro. Run to almost the end of the area to agro all of them, and jump on the overturned truck. Kill all of the zombies and then exit the area. Run back since the zombies at the back should be respwaned now, if not, run out and back in. Drag them to the pile of zombies you have just killed and drop the explosive mine. If all the bodies and the zombies you just killed are within the explosion radius, you will complete the achievement.
This works anywhere else with great amount of zombies but I found it easier at the station.