Step 1: 6star your campaign farmer 

And?

I'm already 1/2 of the way there

Step 2: Farm 12.3 brutal solo with 3 chickens until you have chickens for your next 6star 

How am I supposed to do this if I can't even complete 10-7??

Just do it?

You don't have to get 3 stars in 10-7 now. Getting 1 star is enough to pass that stage. Come back when you have two lvl 60 champs. Much easier to get 3 stars in campaign if your champs are lvl 60, isn't it?

I am about where you are, I have completed all of Brutal except for one sticky one, 11-7.  I have no L60's, have been getting by with 4 L50's.  But to get 3 stars you can only use two champs at best.  Obviously you have made it this far with your two champs, so you can try improving them best you can.  Add some masteries, at least the first two rows, and level up some artifacts past L12.  Apply glyphs if you can, basically just chip away little by little until you get enough to beat those levels.  If you have multiple auras try switching them around.

Your Galek is a bit low on resistance and acc, you might have to switch to another artifact or two to get more balance. Level your ring up, and see if you have an amulet to apply, maybe visit the spider if you can to win one.

You a ways off from solo, so just peck away at it and do what you can, it is not easy.  What is the other champ you use aside from Galek?  You will probably need another L50 at least. Look to see if there is an affinity preference for that one campaign level you are after.

Jizoh and Frozen Banshee are probably not your best choices right now, strengthen your champs that have high rating for campaign.  The game will throw lots of junky champs at you, check them out right away to see if they be worth developing, especially newer ones you never heard of.

The crowd audience mean well, but if you are like me you want to be convinced you did your best without going crazy, so give it your best shot. Good Luck!

1: I only came this far bc of my Samar's sheld

2: Yes, they have both got both rows of masteries; currently farming gems for full mastery.

3: I will try to increase his acc and res

4: I use Samar Gemcursed; he is the legendary dwarf

I am not familiar with that Samar fellow, but here is what I could find:

https://ayumilove.net/raid-shadow-legends-samar-gemcursed-skill-mastery-equip-guide/

Smh... 

Trips was being snarky, but he's not wrong. Get your main campaign farmer to 60 and fully ascended / masteries, and they should have no problem solo farming with three food.

I know, but he said I should farm 12-3 brutal which I haven't even unlocked yet

I think he was getting at saying that you shouldnt worry about 3 starring it for now, but getting to 12-3 even if that means 1 starring just to be able to farm food better and then come back  to 3 starring the harder stages after farming up champs in 12-3

In this previous post, he said "farm 12-3 with 3 chickens" How should I do this if I can't even complete it with 2 champs

You can't do it it with two champs, because your Galek is not lvl 60 right now.

Go back to a stage where you can farm with 3 chickens, 9-3 is quite easy, as all champs except one force in wave 1, have spirit or void affinity there, good for Galek. If you can't find a stage in brutal campaigne, go back to 12-3 hard. The main thing is to make your campaign farmer lvl 60. That's the first and most important goal for further progression.

When your Galek reaches lvl 60, he can farm 12-3 brutal with 3 chickens. When you made a partner for him lvl 60 as well you can get your 3 stars in all campaign stages. Some are much harder than 12-3 to do with only 2 champs, as they are either boss stages or have bad affinity for Galek. 12-3 is easy for him alone.

what does that mean to farm with chickens?

Just means trash champs that you'll be using as food. Usually uncommons.