I'll treat this as a genuine request, even though 12M Brutal can be a troll with this roster :)

Roshcard the Tower + Maneater is an unkillable team. Painkeeper can be used in it too, but probably not required.

The rest are damage, Rhazin, Tyrant and Frozen Banshee are fine, but there may be better comps. Fayne may be good. But this is for later optimization.

First, set up unkillable team (search for guides, videos, how to set up and speed tune it).

Then improve amount of keys you use for Ultra Nightmare (forget Brutal).

Sunny has a point about the unkillable, but that's a project that will take some time - I think in the short run it must be possible to seriously improve your damage if you're really doing that little on Brutal with those champs all maxed. 

Firstly, if you need to use FB's measly 35 accuracy aura, it means your champs' own accuracy just isn't high enough - with high enough accuracy on all champions, you could switch to a more useful aura like Tyrant's. 

Then, they may have maxed masteries, but are they the right masteries? It is kind of hard to see how you wouldn't be doing more damage if your five champs all had warmaster/giantslayer - unless your FB is just bad at putting poisons because of low accuracy? 

Or is it a question of your whole team dying very fast, because their gear is just bad, or they have too offensive gear with not enough defensive stats? 

I would advise to put Drexthar Bloodtwin as the team leader.

How about this:

Drexthar Bloodtwin (leader with +70 Accuracy Aura) - HP burn on a 3-hit A1 and passively on being hit,  massive damage increase, get him as much DEF and HP as you can. As a bonus, any ATK substats will still add to the damage for him.

Frozen Banshee - poison - get her as much HP as and SPD as you can. She also needs ACC, but Drexthar's Aura will help you a lot.

Tyrant Ixlimor - more chance for HP burn, DEF based, has Increase DEF, will tank some damage with Ally Protection, he needs a lot of HP, DEF, and Speed

Scyl of the Drakes - AoE healer, single target reviver, also deals nice damage as she is a DEF based champion, get her as much Speed, DEF and HP as you can.

Fayne - Decrease DEF (3 turns!) and ATK all in one neat, albeit a bit squeeky, erm... squishy, package, get her as much DEF and HP as you can.

Dear Colleagues,

Thank you very much for your answers. You are right - I should at least show you those champions' stats. Here they are:

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Why is it a project that takes time? What takes time there?

Here is what I found on this unkillable: https://www.youtube.com/watch?v=Wq5HVwq-rI8

251 speed on one champ is the only somewhat difficult requirement.

And something needs to be done about stun being it block debuff, cleanse or providing a stun target. It needs research and speed tuning.

I don't understand why spend time and resources working on a team that deals good damage on Brutal when you can make a team that 3-keys UNM.

If for some reason you want a "killable" team, consider Steelskull, he will help keeping a team alive.

My Maneater-Painkeeper unkillable took weeks and frankly getting them to 60 and with enough speed was the easy part, though time-consuming. The difficult part was finding the right balance between def and HP on them as well as on the stun target so that the CB's first stun would go to the right target. And even now it can fail sometimes if the CB crits on the wrong champions in the first two AOEs. I guess I don't know if the other unkillable setups are as delicate as this one though. 

Okay, so you have some gearing issues. Some champions, especially Tyrant, are very slow. You have FB built all offensively, which will add very little damage in the greater scheme of things, instead of having her defensive so she'll last longer and can keep adding poisons for longer. But still I don't think it explains why your damage numbers are so low... what about the masteries, do you have Warmaster/Giantslayer on your champs? Could you maybe share a screenshot of a CB run result, that could also show quite a bit? 

I wish I could go through all this trouble :)

I know I would have used DeadwoodJedi's calculators and spent as much silver, keys, time and paper as required if I had these champions. It's a one time job, it's not like you have to repeat it every day.

But that's me. I can accept that someone would prefer an easy Brutal/NM team to a difficult UNM setup, but I can't imagine myself making this choice and fully understand what drives it. And I can't really think about other possible teams with given roster when I see this one.

UNM drops 5 times more sacred shards that NM. Don't know if Brutal can drop them at all.

Building unkillable depends on the thread starter's gear depth. 

It can be done in a day if he have the artifacts needed. 

Ok, I am attaching CB results and masteries of the champions:

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Yeah, so for champions who you plan to keep using on CB, you should have warmaster/giantslayer - not doing so is just a waste of several million damage. 

And FB should be doing double that amount of damage, if she successfully applies her poisons for long enough, plus warmaster procs. I guess she just dies quickly because of her super-offensive gear? 

You also need to keep in mind that you don't have a serious attack down, which affects your team's survivability too. RT doesn't qualify, since she puts the weak version (reducing CB's attack by 25 percent instead of by half), and with a relatively low chance to apply at that, even if booked. Considering that you have Arbi, I think I'd recommend taking RT out entirely and bringing somebody with a strong decrease attack on their A1. Crypt King Graal might be an interesting option, since he also has a HP burn that will add up to good damage as well. 

Thank you very much, guys. I will try to use your advices. But now... I have to wait for keys restoration :D

  pedyma has a solid a1 tk down. thought id toss that in there.

i am about the same damage on brutal cb so this has been interesting.