I would probably do Frozen Banshee if you could use some progress on your CB team. She can dish out a lot of damage, especially since you got Valkyrie!

You should replace Elhain with Athel in your Arena Team. Athel can buff her own atk and just deals a lot more damage with that move than Elhain does.

After you have done that, look if the opponent teams often survive your first hit or not. If they do, you could think about bringing High Khatun to 60 for more survivability in the battle. If the enemie hits back and she survives she could use her turnmeter decreasing AoE attack in her second turn. If the enemies are wiped out anyway from your first big hit, Khatun can stay at 50 (she could even be lvl 40, it wouldn't matter).

In this case I agree with Benzoboy to upgrade Frozen Banshee, wich will increase the damage of your CB team a lot.

Frozen banshee, maus mage 

Yeah but Frozen banshee is so weak for a tack. I would go with the Mausoleum Mage he is good for the Doom Tower

yes maus mage way better overall, do u have books for mm?

Thank you all for your comments! I just pulled a couple more contenders. Golden Reaper, Dagger, Doomscreetch, and Mother Superior were the notable ones. Should I consider any of these as a vs against FB or MM? Also, I do have 13 epic books I've been holding on to.

I agree with BenzoBoy comment

1 - Frozen Banshee can help your Clan Boss team do more damage.

I agree with Skadi comment

2 - You should use Athel instead of Elhain in Arena.

3 - You should 6 star Frozen Banshee for Clan Boss.

I agree with Trips on his first comment

4 - After 6 staring F.Banshee, M.Mage should be your next 6 barring you don't pull any other heroes.

I disagree with Vdelamorphis comment.

5 - F.Banshee isn't weak.

I use Frozen Banshee in Hard Mode Doom Tower.

I disagree with Trips second comment. 

6 - M.Mage & F.Banshee do different Functions.

I wouldn't say one is better over all vs. other.

They both are equal powerful for their respective functions.