What specifically do you need help with on campaign?  You've got plenty of champs there so should easily be able to 3 star each of them.  If it's a farmer, then you have both Kael and Bellower, so you have options

With regards to CB, what level are you currently at and what is your team?  I see a few good champs floating in there but keen to know how far up you are.  The same applies to your current dragon and spider  

Im at s8-3 on nightmare campaign and im stuck. Farmers i have bellower doing fine. 

CB im running brutal kael, stag knight, apoth, doompriest, frozen banshee. Damage is 16m-

Spider im stuck at s12 with: stag knight, bellower, kael, apoth, and sinesha. In like 3min.

Dragon s15: stag knight, kael, frozen, bellower, lightsworn. In 2,3min

How many keys is 16m Brutal?  Some thoughts on CB are that you are:

• You are missing some defensive characters to help you sustain the fight.  Something like shields, increase defense and ally protect.
• I do see that you have Versulf who can bring two of those to the table.  I'd swap out Doompriest for Versulf, and 6 star him.  He will be useful in a lot of content.  Ideally you want to book his A3 but I'm not sure if you have that capacity. 
• I assume everyone on your CB team have Giant Slayer or Warmaster masteries.  
• What gear do you have on your CB team?  I can only see Lugan highlighted above and his speed is +24, which is horrendously low.  Every champion needs to have a minimum of speed boots, but also other gear that provides +speed.  NM CB has 170 speed, so you will want your champions at least around the 180 mark, if not higher.

i'd suggest making aox your next point of focus. he should be able to add a lot to both your cb and dragon teams. ninja is also an amazing damage dealer, so i'd put him into cb as well. with both of them, stag knight is largely made redundant, so you can drop him and likely either apothecary or kael, depending on how much healing your team needs. you also might want to consider broadmaw. while he's far from the best champion in the game, having a reviver to hand makes things much easier.

also, don't bother with nightmare campaign for the moment. it's designed for players farther along than you are and it's really a one and done, since there's no reason to grind on it. focus on other areas of the game and come back to it once you have a stronger roster.

I totally agree with you about nightmare campaign.

But I disagree on your CB-suggestion, removing Stag Knight. The op is actually running Kael, Stag Knight, Apothecary, Doompriest and Frozen Banshee. Stag Knight is the only one in this combo with decrease atk. He is not the best champ to bring out that debuff at CB, but completely without decrease attack, the runs are done a few turns earlier with less damage. On the long run he should be replaced with a better atk-down champ, but I would never do CB without decrease atk (exept full unkillable teams).

Ninja should join anyway, instead of Kael or Doompriest. She removes the stun, but the CB uses this attack with a cooldown, I don't think the removal is that critical to CB-fights.

So Aox has a Decrease Attack, as well as healing, debuff extensions and an occasional poison. Lightsworn has Increase Defense and Decrease Attack, as well as a throwaway Revive on Death.

I might run Aox, Ninja, Frozen Banshee, Versulf and Lightsworn, as it's very nice to have Decrease Attack up 100% of the time. Then the job is to keep some combination of ally protection and increase def up, as well as permanent dec attack on the CB. I would put your team in lifesteal or bloodthirst, and that will give Frozen and Ninja time to do their work, and more turns of letting HP burn and poison tick away. 

Once you can or if you can get Aox and/or Lightsworn running 4:3 and not taking the stun, and you've booked down their cooldown, then it gets even easier to keep Dec Attack up.  You can then play around with High Khatun for turn meter and increase speed, or Doompriest for stun and affinity cleansing, especially on Spirit. 

Oh, I really missed that Aox has decrease attack, so he can replace Stag Knight. My fault, especially because I have Aox on my account. But there are so many heroes in the game allready, I don't have the stats and skills of everybody in mind.

I'll have to disagree with you on that last point. Nightmare 12-3 / 12-6 is a better return on investment for energy per XP, assuming you can solo grind it with three food champs.

nightmare 12/3 offers about 1000 extra xp per run as compared to 2 runs of brutal 12/3. in exchange, it takes longer and grants almost 3k less silver, not including the loss you'll likely take from only pulling half the rewards. so while it is better strictly in terms of xp, it's still a worse propect overall.