I would go with Deacon, speed aura, turn meter manipulation and AoE decrease defense. Would work well with Athel. Or you could go Scyl, but I would still go Deacon first. 

You made a classic mistake that almost all new players (including myself) do when they start playing. Instead of all those 50s you should have 3x60 instead.

As  Arcllite  said,  either  Deacon  or  Scyl.  Personally  I  would  go Scyl  first  and  then  Deacon.  Both  can  still  be useful  at  50  so  in  the  end  it  doesn't  really  matter  which  one  you  pick.  I  only  say  Scyl  first  because  she  enabled  me  to  break  through  the  wall  I  was  stuck  at  because  of  her  stun  chance,  passive  heal,  and  revive.  

I would go scyl, she is useful in every single dungeon, doom tower, tag team- solid champ and would 6 star asap.

I'd go with Deacon. Scyl is nice, but honestly, she sits in my vault most of the time.

Deacon first, due to his AoE decrease defense. That is one of the most important abilities to have on a team to clear content. After that, I would get Scyl to 60, she is a hard carry in dungeons and doom tower.

THANKS ,,,, Deacon was my first thought,, 

but like some said  Scyl was bugging me because I use her lots

But I have noticed a Nice boost in a few dungeon lvls with Deacon ( Spider and Keeps )

CH was tickling my brain too , But YA,, Deacon then the rest will come easy ..........

and then there is DoomPriest .....

Don't  listen  to  her!

I don't see deacon on your dragon 20  team.  So I would go scyl.  You have lots of 12.3 brutal in your near future as you work on a dragon team.  As someone else said, too many 50s and lots of poor choices... 

Glad  I  didn't  show  you  my  account  when  I  was  growing  up.  

According to your roster I would suggest Scyl should be upgraded to six stars. Scyl helped me alot to keep my various dungeon teams alive and to consistently farm Dragon's Lair stage 20!

Scyl or Deacon. Scyl will carry most teams through pve amd deacon would help a ton in arena! :)

I would 6 doompriest. She requires no books to make a major impact. Shes vital through out doom tower. She can run all the way in ultra nightmare. Shes great in dragon useful in every dungeon.