Join any multiplayer game and wait until you are on the defending side. To place a barricade, walk up to any open doorway or window, then press and hold
. You can unlock this by placing five barricades in five separate doorways/windows. Or, if you're like me, you can tear down a barricade and replace it four or five times consecutively, Your teammates may look at you strangely at first, but soon they will bask in the glory of your prestigious 20G achievement.
Please feel free to message me if I can make this solution any better. Otherwise, happy hunting!