If that's your entire roster, then I wouldn't stress too much about it. You've too much you need to develop still, before being able to start thinking about CB-exclusive champs. Just put your five best-geared champs on the team and get as much damage as you can.

CB teams require champs that are geared specifically for it, because you need their speeds to be set to specific levels, and those speeds are often too slow to be usable anywhere else.

You're definitely gonna have to do some trial and error. Force-closing the game during a run should get you a key back.

The main issue you're facing is you don't have the champs to build one of the Unkillable style teams, which is the way a lot of newer accounts have to go to get their UNM damage in 2 keys. If it's taking you THREE keys for Nightmare, I hate to say but you're pretty far from getting UNM. You need to get your NM runs down to 1 key most of the time and 2 keys sometimes, depending on affinity.

All that being said, based on your current roster, I have 3 suggestions:

1. Try to fuse Keeyra soon as possible. She's part of Mikage anyway and you have a lot of the champs needed.

2. Build Alice and stop putting work into Skeletor for now

3. Your team to try is Geomancer, Tagoar, Stag Knight, Kael and Aox.  Geo is your hp burn and some survival + weaken. Stag knight is your Dec Def & Atk. Tagoar is your best reviver that you have built and does heals + shields + speed. Aox and Kael have poison synergy and try to get Aox in your best Retribution gear. His A2 might get some big heals with enough debuffs on the CB. Replace Aox and Kael if you can.

That team, as long as it is built for survival and not dps (since all the dmg comes from poisons, hp burn and warmaster) MIGHT, MIGHT get you a 4 key UNM

On a quick roster check there is go: MM for AP Deakon for Dec Def/atk Doompriest for cleanse/healing Geo for DPS ninja for DPS

Kael instead of Ninja might be better for you ATM Same on Apothcary instead of doompriest Anax is top tier damage but might be too hard to keep Alive with the rest of your roster

 https://omegle.onl/ 

I think 2 key UNM would be tough with your current roster but NM should be doable without too much trouble