big  un  for  sure, gorgorab  is  a  good  speed  lead  in  arena  and  should  be  fine  at  50  if  you  are  strict  on  resources.

Yeah, hard to argue with Big'Un... even though you might need support more than offense right now, he's ridiculously good. Definitely Maus Mage or Gorgorab after that then. Spider 20 may not be feasible with the roster you have now, but dragon 20 could be, although you're still missing a good aura. 

Also, I'd suggest to push back your goal of clearing nightmare campaign until, uh, a good ways into the future. That's the kind of endgame stuff you do after you've finished all dungeons stage 20, you 3-key ultra nightmare clan boss every day, etc. Instead, your goal should be to progress on CB. 

Thank you guys, Big Un it is! I was thinking the same thing but wanted to get some advice before I applied it. I'll focus on a few others mentioned next up.

I vote Narma the Returned because she can do Attack Down for your Clan Boss team.

Thank you Player J - She is pretty awesome and will be up for a 6 star soon. Hope your doing well.

Narma (even unbooked) is way more versatile than FB.  I recommend 6starring as well