You have some truly great champs. Lucky!

Minaya, Roric, Bivald, Gorg, Serris, Khatun, Venomage, Bellower, and Frozen Banshee are all worth building up.

However, of them, I think Bellower should be your first to bring to 60. Athel isn't great at campaign farming - getting someone who can farm Brutal 12-3 quickly is very important, and Bellower can do it in 10 seconds or less.

After that, I think Bivald should be your next priority. Getting an AoE Provoke will help keep your team safe. Your Dragon 20 team will likely be:

• Bivald (bad affinity, but it should still be okay, because he can provoke on weak hits) - provoke, shield
• Gorgorab - speed up, heal, revive
• Venomage - boss killer
• Khatun - TM up, team lead
• Bellower - waveclearer

i would probably go lua next over bellower. she won't be quite as efficient as a campaign farmer, but she's still perfectly serviceable. more  importantly, she's far and away your best nuker for arena. after that, my next three would likely be bivald, minaya, and serris (not necessarily in that order).

in regard to your arena team, i would suggest gorgorab, serris, and lua for sure. from the options you've listed here, i think the best fourth would either be high khatun to have a double tm boost or bivald for his crowd control. that said, you may have far better options by the time you're locking in a permanent arena setup.

Gorgorab 

Either way 12.3 brutal is your home :)

Thanks  for  the  write up,  really  helpful. I  dont  have  that  many  epic  books  tho  so  I  might  have  trouble  using  both  Venomage  and  Kathun.  Maybe  I  should  use  Frozen  Banshee  and  Diabolist  until  I  get  more  books?

Frozen Banshee *can* work for Dragon 20, but to be truly effective you'll need to make her pretty fast. The real reason why she's not ideal for that team, though, is because she's really quite terrible at waveclearing, whereas Venomage does the job much better. Still - especially if you have Bellower (or Lua, as Carpe mentioned), you can probably get away with it.

As for Diabolist - yeah she's a poor man's Khatun. No reason why you couldn't run her instead.

don't bother booking khatun. i wouldn't even bother taking her to 60 unless you end up using her for arena. she's a pretty good champion for newer players, but you already have better alternatives for most of what she brings to the team. i'd advise the same for diabolist. both of them might be helpful for now, but there's no point throwing a ton of resources into a champ you're gonna stop using in a month or two.