Legacy of Rust, Episode 2: Counterfeit Eden, Mission 6: Soul Silo
Recommend I'm Too Young To Die Difficulty as it doesn't affect enemy spawn density in this level, just health.
Start the mission and, using a Keyboard, open the map and type IDDT twice to unveil the entirety of the map, including Enemies. Close the map and type IDDQD for God Mode, and IDFKA for all keys and guns.
Head out of spawn and turn left, go through the door past the Shotgunners, and follow the outdoor area around the curved pathway to the red key door.
Enter and hit the switch on the right side just inside the door to open the next door.
Go around to the right past the first column of Toxic Waste, then go left through the Blue Key door, next to the Incinerator weapon pickup on the floor. Follow this corridor all the way to the Yellow Key and go to pick it up.
You'll find you've been teleported to the area in the top left of the map (This is the final arena of the map leading to the end). Feel free to make a Quick Save.
Head forward and drop into the Pit. When you do, all of the Imps that are in the two adjacent pits to the east and west will be raised up to start attacking you.
Type in IDCLIP to enable Noclip and let yourself out of the pit by just walking into one of the walls, and find a spot with the Imps in front of you, where you have some space to be able to fire the Calamity Blade (Weapon 7) at full charge. You should be able to get well over 50 kills easily. If you run out of ammo just type IDFA or IDKFA again. Load your Quick Save if you end up killing too many, there's not much point in waiting for them to respawn with the modifier on.
Here is a quick map path I put together, follow the Cyan route. Starred locations are the Imp spawns.
https://imgur.com/a/JIrWbQ3