Looks like you just need to invest in more Clan Boss specific champs.  You've got great options but they are undeveloped currently.

Skullcrusher and Sepulcher Sentinel should be high on the list to level.  Id recommend a team of something like Skullcrusher / Sepulcher / Rearguard Seargeant / Frozen Banshee / Geomancer to work towards.  

Below is some basic guidance for a speed tune you can setup with those champs.

4/3 Sepulcher with Counter Attack.  Double turn between the AOEs for both NM / UNM without Gear Changes.  Syncs at Turn 7 for both.

Sepulcher: 243- 244
Counter Attacker: 191
DPS 1: 179 - 182
DPS 2: 179 - 182
DPS 3: 170 - 178

UNM - 1 Turn Delay on Sepulcher's A2.  CA champion is vulnerable to the 1st and 2nd stuns, adjust delays as required.

NM ( Void / Magic / Force ) - 3 Turn Delay on Sepulcher's A2.

NM ( Spirit ) - 2 Turn Delay on Sepulcher's A2.  If running Sepulcher as lead she should take the 2nd stun and then be in tune.  If she does not take the stun add in a 1 move delay at turn 7.

https://deadwoodjedi.info/cb/beacb0475ed6099c251840d8b554b08fef9fa252

As an additional note Demytha is one of the best CB champs to make an Unkillable team with but currently you're missing some of the other pieces to really make her work.  Seeker being the most crucial.

Thank you for the strategy! I appreciate your suggestion. I'll give it a try and see if it solves my problem. I'll be sure to let you know if it works. 

id agree with adding skullcrusher to the mix, he should really help add some extra numbers of damage for you.