I have my own map,http://auriga-squad.appspot.com/get/agxhdXJpZ2Etc3F1YWRyCwsSA01hcBjO2y0M
This map is hard due to the lack of bullets when expanding your territory
If your cornored by a whole bunch of zombies without enough time to activate airstrike,then just grab your shotgun,run forward,and rapidly shoot :D.(Use minigun if no more shotgun ammo)
to make walls where regular zombies cant destroy it, just plant a wall when you are standing still and dont move.Wait for a zombie to hit you out of the inside of the wall and they cant destroy it.(devil fireballs and explosions you use can destroy these walls).