A room loaded with a hole in the floor, blocking access to two doors. One door was already open leading to a spawner. During waves, those monsters would run up to the door and not be able to progress farther. 

While this route was the direct route to the crystal, it was an easy thing to then just shoot them. When I moved the crystal, they piled up in a corner of their dark room, trying to reach the crystal through the wall, leaving me stuck in never ending combat as they were out of line of sight.

TED.7z