During the First mission as you start the game from the main menu. Continue playing the game until the tanks show up. Then hijack a tank and use it to run over and blast everyone to gain this achievement.
With all the crowds on this level (or later in the game) you can snag this achievement easy. The crowds can be seen off the main road along the intersecting roads.
However, if you wait until later on this mission to go after it. The enemies will get stronger by sending more helicopters and tanks. So the best approach is to grab the last tank standing of the first group and then do not go near the next waypoint with the Hunters.