So I am not sure how many people know about this, but at least on the PS4, you can go into Circle of Slaughter like right away. I am barely even into the game yet, went to my option screen, went down to social, and then there is the Circle of Slaughter you can queue up for.
I thought CoS was supposed to be an end game activity, only able to be done by people who have beaten the game, but it appears not to be the case, and since monsters scale down to your level in this game, you can go in at any level.
My friends and I did this and we were all level 15, and all managed to get to 19-20 in one game of Circle of Slaughter, and each banked over 200k, which is massive.
Would recommend it for sure for getting starting cash to set you up for most of the campaign.