Me and my friend did this, completed the four sets on legendary, and it wouldn’t give us the 30k credits. Why?
You must begin the fifth set and then the challenge will be completed.
Oh, you poor soul. You have to set the limit to 5 or more sets. Don’t worry, you’re not forced to play regular Firefight, you can enable God mode and it will still count. But after the 4th set, you can safely end it and you will receive your Cr.
I have a custom variation I created just to farm this weekly challenge. Alone with a dummy control it takes about 40 minutes. With a friend, takes about 24 minutes. I keep playing until the 30,000 credit notice flashes on my screen.