1. Dungeons. I think you don't need Skullcrusher in your dungeon team later on. When you have better gear, you are so much faster than the opponents in dungeons, counterattack is not the way to deal damage anymore here. Maybe it's usefull now, but I wouldn't use him in a Dragon team once you have some lvl 60 damage dealers and nuke the enemies away before they can attack you at all.

Do you have a stun set with some good stats? Bellower fits perfect in this set, it brings some control while fighting the waves. If you use his A1 and, from Apo's speed buff, your Bellower is fast enough to go twice before the enemies got their first turn, you have a great chance to take out a lot of enemies. Either they are stunned or reduced to using their A1 skill.

A good team I see would be Apo, Bellower, Warmaiden, Athel, Trunda. 

For Athel and Apo I recommend Giantslayer mastery. For Bellower, who is no bosskiller anyway, you could go for Fearsome Presence (defense tree), if you decide to use a stun set. The mastery increases the stun chance from the set as well. Trunda should have Helmsmasher. For Warmaiden probably the standard Warmaster, if you bring her to 60 at all. I would use Warmaiden at lvl 50 for now, she does her job at this level. Maybe you find a better decrease def champ later on. Warmaiden is only the starter kit for this debuff.

Athel can be replaced later on, if you have better damage dealers. Did you get your Ronda allready?

2. Spirithost (teamleader for her speed aura), increase attack, lvl 40 is enough. Apo, Warmaiden, Trunda. Every opponent you will meet in lower arena should be dead after an attack buffed Trunda hit them.

3. Apo (early on), Skullcrusher and Scabrius are good for CB. Warmaiden and Trunda are not particulary usefull for this content. Warmaiden's def debuff is usefull, but it's on a cooldown and she doesn't bring anything else. Use her for now, but replace her with a better option soon. 

You need decrease attack debuff on the CB. Your team lacks that one. Bellower has only the small version, you want the 50% debuff.

Unfortunately I do not have a stun set, so maybe I put my progression in dungeons on a halt and focus on getting to let's say Brutal CB, but as you said I would really need a dec. attack champ, which I don't have in any kind of form yet, and I haven't found any that I can farm or get aside from shards. 

I do got Ronda, and I plan to build her, but that's just months away from now I think.

For CB I can run Skullcrusher, Apo and Scabrius as you said, and for more damage I can put in Athel for now, and lastly Bellower as the temporery dec. def and dec attack champion