On day one, there are 2 sniper platforms where you have to walk up stairs to get to, each with cycling weapons. One has a mortar and hammer of dawn, and the other has a sniper / torque bow. If you set a Boomshield where the path curves round to the sniper spawns, the Boomers and Grinders get stuck. the only enemies which can get past are the Drones, which aren't very common in the higher levels. even if the shield fails, he Boomers cant climb over the barricade which leads to another area. this got me to level 38 in the horde without dying once.