Go to the building with the mulcher/mortar on top and don't go inside, instead go onto the roof where the mulcher/mortar spawns as there is only two staircases to cover. Make sure you make good use of the mulcher/mortar and in the later stages try to get some shield of the maulers to shield the staircase entrances. Make sure you plant the shield low enough so you don't get hit by grinder fire. You will need at least two people to do it with. One person covering each staircase. However with five people it should be achieved easily.