one key I think, but pretty easy since you can do this in any difficulty

I cannot do 20 turns yet on one key with any level, sorry to say. If I try another key does not seem to work.

Maybe you can post your roster and we help to build a CB-team for you. It shouldn't be a problem to build a team that can stand 20 rounds on easy.

Yea, it's 20 rounds with a single key.  Depending on where you are up to in the game it should be possible to do on the easiest difficulty.  If you post your roster we can try to help

I am answering your question but I had to switch to my latest account, no longer LADYGAGA.  Due to arena collapse I decided to restart the game from bronze 1, since I dropped like a rock out of Gold 4. I am about 15 days in, have one Athel L50, the rest are all measly weak rares I have accumulated after many others discarded.  I just got the free Jizoh from tha game but he is not worth much for CB. At my level you just do not get anything worthwhile, and I am FTP.  No artifacts, no champs, nothing zilch nada zip.  On EASY my best hit is about 800k, maybe 14 -15 turns. My level is 36. So it will be a long slow crawl. I have Shaman, also not great for CB.  I am doing all I can with ayumilove, etc.  It is all the same, I have never gotten good champs like I see everyone else has.  I am still fighting with shaman, warpriest, warmaiden, etc but in the weak L40 version.  It will take forever for me to get a L60.  Still waiting for an apothecary and second starter champ. I have only 2 champs with power >10k, so that's the story, no power, no good skills, no CB.

It is just a slap in the face insult to get blue ancient shards with all the hype about epics and legendaries, but no, not only do I only get rares but the junk 1,2,3 star skills at best.

I will be  honest here

Part of me being able to complete those missions to eventually get Arbiter was building a Unkillable team for CB

Its a ton of research and work but worth it  in the end when you get it done. Also worth researching regular CB team builds that may fit your champion collection best

Its something everyone needs to do to take their account to the next lvl. Good Luck👍

Right now I am just throwing paper airplanes.   Most of my champs do not even have a rating for CB.  Out of the 20 that I have only 6 have a rating that is 4 or 5.  Average around 6k power each, so I am not under any illusion, it is just going to take a very long time.  After about 2 more weeks I will get High Katun, maybe that will help.

The quest to get "50 debuffs on the demon lord" for some reason allows you to use multiple keys to accumulate that.

I struggled with it when i first started playing, you will get there