You go up one staircase and there is a small room with another staircase that leads up to the room with all the Chosen Acolytes and the Hive zombie things. The Hive zombie things charge at you. They will charge at you several times throughout the fight. If you run back to that little room between the two staircases and run into the far corner, it is like they lose aggro and just turn around and run back up the stairs. Then you can creep up the stairs and see them just standing there. Once you shoot, they'll charge again, but you can just keep rinsing and repeating.