Walls count as either walls on a building or the fences around perimeters of bases. While on foot the easiest way to destroy a wall is to shoot it with a grenade from an assault rifle and the easiest way to destroy a fence is simply to knife it.
If you really want the achievement I recommend loading up into a tank simply shooting at every building you see until it’s completely destroyed. I found this tactic was easiest to do on the level “Ghost Town.” At the very start of the level you’ll receive a tank and slightly later you’ll come to an area with a lot of buildings.
This one should come with story progression though so don’t worry too much about it.