Fast Shooter is a challenge in Borderlands: The Pre-Sequel. This challenge can only be completed during the Let's Build a Robot Army mission. Once a certain amount of the mission is completed, the first door will always be open, and the second will already be closed.
After opening the gate to the Stingray Facility, the NPC's can be engaged near the gate, followed by a run to the end with a rocket launcher to kill the lil' scavs before they close the gate.
The gate is at the far end of the facility. The mobs must be engaged at the beginning near the gate opened for the lil' scavs to spawn at the far end.