To get the Flash Mob achievement in State of Decay 2 do the following:
Hordes are shown on your map and mini-map as two green walking zombie silhouettes. Driving around your region will spawn plenty of these frequently. The easiest way to eliminate them is to keep on driving -- as in, right through them. They come in different sizes, seemingly from 3 to 10ish zombies at a time, and you'll know you've eliminated them when they disappear from your mini-map.
When you've killed 24 more Hordes you'll also unlock the following achievement: