For starters, you should never, ever use champs that are not level 1 as food. So if your four others that you plan on feeding to Elhain are level 40, do not use them as food. Save them for later, when you can rank them up to 5*, and then use them as food to get a champ to 6*.

With respect to your specific problem - shouldn't be too difficult to just farm a lower difficulty for a few levels, until your Elhain becomes strong enough to farm the stage she used to farm for you.

you should have plenty of brews hopefully, use them to get her back to lvl 20 or so, then you should be good to use her again atleast for lvling her further in campaign

otherwise got to do it the slow way in campaign on lower difficulty stages

p.s. you should never use brews except during fusions or in this special case, because they help a ton during fusions to reach champ training goals

So only my lv. 1 galek is eligable for getting eaten?

Oop all my brews are in the stomach of my high katun

WOW you have no clue what your doing ....

You lvl up 2 Star GREEN UNCOMMON as FOOD ,,,,

You do the Dungeon over and over and over and ovwer untill you lvl up the UNCOMMON GREEN 2 STAR JUNK to 4 star lvl 1 and FEED THOSE to your heros ,,,,

TRY READING THIS >>> https://ayumilove.net/raid-shadow-legends-guide/#do-and-dont 

Then READ IT AGAIN !!! 

THEN READ THIS STUFF TOO ............. https://hellhades.com/raid-shadow-legends/    

THEN REFERENCE HERE FOR A FAST ROUGH GUIDE so you dont lvl up trash  

I'm not planning on feeding my fusion uncommons or saurus and armiger any time soon..

And my outlawmonk isn't going into any elf's belly

No not those ,,, 

       your not Listening or Reading are you ,,,,,, 

You get 100s of Green uncommon Junk from Green Shards ...

You get them Free from Campaign  ... 

Now Maybe go READ A BIT HERE find out Whats TRASH,, Whats Decent ...

 https://ayumilove.net/raid-shadow-legends-list-of-champions-by-ranking/ 

SEE JUNK FOR FOOD 

i

You get more than enough of all of the 2* champs. You don't need to save any. And you shouldn't devote any resources to leveling those, except as food - with the possible sole exception of Armiger, although even there, I wouldn't waste time leveling it right now.