Brutal 12-3 is the most energy efficient overall for silver and XP. 

Shields sell for more silver.  Shields on level 3 of any campaing chapter.  6 is a bit more experience.  So it makes a difference if you need more silver/experience.  But overall the silver boost is better than the experience advantage.

12-3.  Shield.  Sells  for  the  same  or  less  as  selling  12-6.  I  dont  get  it.  Very  little  difference.  How  do  you  know?  When  i  play  i  auto  battle.  No  way  of  knowing.  But  not  auto  battle  i  sell  anything  not  5  or  6*.  12-3  gives  very  little  coin.  12-6  gives  very  little  coin.  I  dont  notice  anything  thats  enough  to  pick  12-3  or  12-6  when  12-6  gives  more  xp.

i  dont  spend  much  coin.  I  dont  have  many  5-6*  artifacts and  dont  know  who  to  use  with  who  and  masteries  and  what  artifacts  to  use  except  i  think  divine  is  better  then  non.  

i  level  up  food  to  upgrade  my  40s  and  50s.  

ugh  :)

No way of knowing?  I do not recommend you take my word for it.  I recommend you stop playing on auto battle long enough to take notes and write them down.  But like I said, which is better really depends.  Are you silver poor? Or will the experience do you better?  I get 50 multi-battles per day and would never waste them on campain.  When I'm grinding for food I can easily do 200-400 runs per day or more, so multi does me no good there.  On 12-3 the top item is worth 25K silver.  I don't recall what the top item on 12-6 is, but it's not 25k.  We are just in different stages of the game.
Cheers!

Maybe you compared a shield with less stars? 3* commen shield from 12-3 sells for 5538. 3* boots from 12-6 sell for 4985. The difference is small, but the difference in xp is small as well.

Another reason for 12-3 is, that it's so easy to do. 11 of 12 enemies have spirit affinity, only one enemy in wave 1 has force affinity. A champ with magic affinity is strong vs. 11 of them, easy fight.

I  did  compare  low  *  like  3*

This is just one of those things that's solved in the game. 

For XP: 12-6

For silver: 12-3

For overall efficiency and ease while taking both silver/XP into account: 12-3

I'm not sure what you're asking. Can you be more specific?

im sorry im confused. spend more coins on what?

I'm not really certain either. Hopefully they come back and specify what they meant. :)

imma dm them so we can get the answer quicker.

k now we wait

If you want more credits you farm 12-3 on Brutal.

If you want more exp then you farm 12-6 on Brutal.

Kael does both of those very easy. So you can use 3 "food" champions next to him.