Fighting the Club Boss is a group effort. To earn rewards you need to fight the Boss at least once per day. The Rewards shown in the Club Boss are only awarded if your Club successfully defeats the Boss, but you will still get some rewards if you don't make it past the Boss!
If your Club defeats the Boss
If your Club defeats the Boss, you will get the rewards for the rank you reached that day (A, B, C or D). Tap on the Rewards icon in the upper right corner of the Club Boss screen for more details. A rank A result will yield higher rewards than a rank B, C or D!
If your Club does not defeat the Boss
It's alright, it happens to the greatest Clubs! While you won't get the rewards shown for each rank, you will still get some rewards to help you work on your strategy. All Club members who fought the Boss will get the same rewards in that case. Note that Diamonds are only awarded if your Club successfully defeats the Boss.