This can be done on any difficulty but prefer to playing on easy. Make sure that all the tiles are in-range of signal repeaters and can be used. My strategy was spamming laser towers in late game. You can refer to using cannons or arrow towers just make sure there are walls near the borders of the map so you dont have to worry about zombies coming in. Economy should be at least 10m/s so you can finish the achievement faster. Once done, wait for the towers to build, and done! You have the achievement!