If you want to use stag knight as your only dec atk, he should be fast enough to reapply the debuff every 2 turns which means he has to run in 3:2 ratio that is with at least 258 speed (dont forget he also needs accuracy to land debuffs). However, remember that every skill has 3% chance of being resisted, so its unadvised to use champ with decrease atk on skill other than A1. You got lightsworn - def atk on A1 and inc def, which is perfect. But maneater with 100% crit rate is 100% sufficient for dec atk since he has 100% chance to apply it when he crits with his A1.
Zavia is pretty good poisoner, but she has low def and HP, whereas Occult brawler has a bit better stats and will work perfectly with skullcrusher's counter atk. Frozen banshee is also great option with skullcrusher.
As mentioned above, if you want a maximum dmg you need CB champs to be lvl 60 with warmaster mastery.
The composition I suggested is something that you should aim for = skullcrusher, fayne, maneater, occult brawler + 1 dps with def up ( iron brago or altan would be perfect, or something similar - lightsworn, sepulcher sentinel or grizzled jarl).
Other than that you have everything you need to make unkillable comp with maneater and warcaster and skullcrusher, which is the best and pretty accessible comp for cb. (although I saw a video of a team making 200M dmg on UNM, but the team had 2 x krisk and valk so it should be forbidden xd)
I know its pretty long run for you, since you have to lvl them all up but well. It will be worth it, since UNM CB is the best source of books/lego shards.
Overall you need champs that have:
1) dec atk, dec def and weaken - these should be up all the time to make you survive much longer and deal much more dmg. This is the highest priority.
2) poison, counter atk - for even more dmg. poison is the best and easiest way to deal dmg to cb.
3) inc def + ally prot - for better survival.