Your team has to many redundant debuffers. You need one source for decrease attack. Riho and Stag Knight and Coffin Smasher are all good for CB, but not together in one team, it's just a waste of team spots. The CB starts his turn cycle with 2 AoE attacks in a row, then he does his single target hit that stuns. The stun hit is NOT based on his attack, it's ok if the debuff is not active in this turn, you only need it vs. the AoE attacks. Riho alone is able to keep the decrease attack buff up all the time you need it. 

Riho, Nazana (ally protection, shield), Ronda, Kael, Venomage. All champs in CB have to be lvl 60 and need Warmaster/Giantslayer tier 6 mastery. I doubt that the team I mentioned, or any team you can build right now, is able to do UNM, there is a lack of important buffs and debuffs. Unless your team is unkillable, you want to add increase def buff and leech debuff. But I dodn't see any champs for that in your roster. 

Thanks, that's helpful. :) I know UNM is a while off, hoping to get a maneater at some point so I can start building an unkillable team.

Just a "side note" so you don't end up doing what I did... 

Keep Heiress. Her A2  removes the CB Stun & gives +30% speed boost both on a 3 turn cooldown w/o books (you will need to book her to get the remove debuffs to 100%). If you happen to pull a couple of other champions she is amazing in an unkillable team. Even if you don't level her now you may end up needing her later.

I used several of copies of her as food. Once I realized I needed her for my CB team... took me FOREVER to farm another one.