It  would  probably  be  Dhukk  the  Pierced  or  Aprgecary  due  to  their  overall  utility.  Who  is  your  12-3  Brutal  campaign  farmer?

First criterium for your first 6 star is to have someone who can do campaign farming in brutal 12.3 - as well as adding value elsewhere. Clearly you've chosen not to go the usual route and 6* your starter, which considering your two legos might work out indeed... Astralon seems like the better choice given that Bloodgorged is weak affinity for chapter 12 of campaign. 

Dhukk just might be the best champion you have though, his skillset is crazy good, so he'll be next on the list then. 

I can farm 12.3 with Gala in a lifesteal kit atm. She is the only one I have fully ascended and full masteries. She does get defeated every now and then though. 

If I continue to use Gala as my campaign farmer should I take Dhukk to 6* first you think?

I'd say so, yes. He could probably farm campaign too, for that matter. Not sure on the order after that, I'd still think Astralon, then Bloodgorged. Apothecary should get to 6* at some point but you're not really in a hurry on that, he can do most of his job just fine at 5*. Fellhound is mainly used as FK specialist and as super-fast campaign farmer - but unless you somehow have limited time to play and find fast farming very important, a speed farmer isn't generally top priority either. 

I am new to the game and I see it says you can upgrade your champion to 6 stars,  I find that impossible.  I have been playing foir 2 weeks and can only get Kael to level 4.  What am I doing wrong?

You  have  to  get  to  the  max  levels  of  the  rank,  and  get  the  amount  of  food  champions  or  chickens.  For  example,  to  get  to  rank  5,  youll  need  a  4  star  level  40  person,  the  person  your  upgrading  their  rank,  and  four   fourstar  foods.  To  get  to  rank  6,  youll  need  a  5  star  level  50  champion,  and  five  five  star  food

@Dracaras Just to clarify on the above - you'd take 2* uncommon (green) champions, level them to 20, then rank them up to 3* by sacrificing 2 other uncommons. Then level them to 30 and rank them up to 4* by sacrificing 3 other uncommons who you have also increased to 3*. And so on. 

In other words, getting your Kael to 6 stars will require hundreds of uncommon sacrifice champions - yes, it's a big grind. You may be tempted to take shortcuts by using rare (blue) or epic (purple) champions instead of uncommons - I wouldn't do that unless they're duplicate rares, or rares that drop from campaign and you don't plan to use any time soon (because you can always replace those relatively easily if you'd need them later). 

Awesome man.. Thank you for your help!

I would go with Astralon