A  complete  list  of  who  I  can  work  with.

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Definitely focus on Big 'Un. Just gear him with as much attack, crit rate, and crit damage possible. I'd also recommend to buy him masteries if you got enough gems saved up. That would certainly help with finishing Brutal Campaign. 

Maybe use Sinesha at 5* as support/ additional damage with Big 'Un on Brutal 11 stages, but Apothecary+Big 'Un should be able to get 3* on everything else. Good luck! 

I would say Apo for sure, you need the support around your damage ealer.

I recently pulled Sinesha and not built yet, can't say how she would help, but know she can do damage as well as support, and being Force may be a great option as well.

Couple of tips: 

You should build up your Armiger, use extras from Mystery shards, or market, as books. He will help you when you get to the mid ~ end game. Easy/cheap champ to build up. You have time, so not a priority now, get your Dragon team and CB teams set up, and slowly can get Armiger to 60.

Also, take a look at champs you need for the different fusions in the Portal, Rhazin, Relic Keeper, etc. If you have them lock them up, and dont use as chickens to level other champs.

Big Un is a hitter , max him out with , books, masteries, artifacts, accesories , he is good fully loaded.

Apothecary is good for turn meter manipulation

Coldheart can make the spider run away pretty quickly