I was unsuccessful using the other methods. I found a method that worked for me and hopefully some others that may be struggling.
You will need to play on "I Am Death Incarnate" difficulty (at a minimum) and achieve a score of 17,200 points.
This method is not effective 100% of the time. The AI need to come at you in a sequence that will give some combos. You don't need a lot of them, but some are mandatory. As you see in the video, run left and harvest some supplies and get your guns sorted. Take your time. The AI will not mess with you, and it takes a few for the stairwell to clear out. As you can see I arrived a little early. Once you get past that part, it is really easy. It just takes some luck to get the AI right, and headshots/explosions only. I was able to reach gold about 1/3 of the time this way.
Gilbert Prime comment:
I recommend at the start (the 6 second mark in the video) instead of immediately heading around the edge of the room, pop into the center and throw a knife at the civilian shambler's head for an extra 700 (300 Shambler+200 Thrown Knife+200 Head Shot) because you will need every point you can get on this challenge.