You should attempt building fifty barricades simultaneously.
A handful of people are reporting that they are having trouble unlocking this through matchmaking. However, they are able to unlock it in a private match. I did this in a private match with one of my friends and it seemed to unlock no problem.
As you build barricades and bases, turret placements will become available. In my opinion you should attempt this later in firefight and have friends. Also you should be aware that this achievement is not cumulative
across players either. So all I can really offer are recommendations for this.
-Do this after you have began to accumulate large amounts of supplies.
-You should not upgrade any turrets/barricades, it is not only a waste of money while going after these achievements, but will allow you to build turret/barricade after destroyed.
-Have your friends let you build on their barricades and turrets.
-Having mainly air units is an excellent way to get around the map and make this achievement simpler. As some of your own barricades are not traversal by your own ground units after built. This is especially true given you will probably be building up from the inside-out of the map blocking yourself as you go along.