Mad Max

Scarecrows are one of the multiple forms of threat you'll find all around the map. They are tall structures with fire at the top. You'll be introduced to one very early in the story missions. This achievement requires you to take down all of them across all regions. The only way to get them marked on your map, aside from stumbling across them while driving around, is to view them from the balloon of that region's Vantage Outpost. They're relatively easy to spot, but if you find that you're missing some in a region where the map screen tells you that you have so many left in that region but you don't see that many marked on your map, use THESE IMAGES over on Steam. They show all points of interest for all regions, allowing you to quickly figure out where your missing scarecrows are.

In terms of actually destroying them, there are three types of scarecrows: weak, fortified, and massive. There are also three ways to destroy them: ram them with your car, pull them down with your harpoon, or shoot them with your Thunderpoon. Weak scarecrows can be destroyed with a simple ram, the base harpoon, or a single Thunderpoon shot. The fortified ones require an upgraded harpoon to pull down, but can still be rammed or shot, just with more effort. The massive ones require a fully upgraded harpoon, or even stronger ramming or many Thunderpoon shots. Since it's highly unlikely you'll have enough harpoon or Thunderpoon upgrades to take down the fortified and massive ones when you come across them, I highly recommend ramming as many as possible. You'll find that it's far more efficient than wasting tons of Thunderpoon shots, especially on the massive ones. That being said, you can definitely feel free to wait until you have the fully upgraded harpoon before bothering to destroy all scarecrows, but ramming works just as well.