It means survive 20 turns against the clan boss. 

In the battle it shows how many rounds.  I never survive on UNM.  Do I need to fight at a lower level where I wont and the CB dies?

In this or any other instance if it doesn't specify a difficulty for completing the objective: yes, that would work. :)

I got enough turns against Easy CB.  Thx for the info.  

This thread is raising some red flags. 

What CB difficulty are you using your keys against? From your earlier post it seems it would be UNM, but if you aren't able to survive even 20 turns, I very much doubt you're able to get even the first chest, let alone the top one.

Conversely, you then post about doing Easy CB. That's the complete opposite end of the spectrum. 20 turns should be absolutely effortless for any team that is getting top chest at any difficulty.

What's your CB team? My guess is that you should be focusing your efforts on Brutal, but hopefully eyeing Nightmare CB - and ultimately, UNM yes, but let's walk before we run.

Smh...

It's a bit troubling because I'm fairly certain @ClosedPoly has a roster that should be able to do Brutal/Nightmare even if gear is a bit lacking. I would be curious to see an updated roster, though.