That's something I wrote in a similar thread:

None of the allready levelled and ascended champs are food! Don't waste the effort you put in them.

Buy a random uncommen at the market or open some green shards and start levelling up uncommen champs in campaign. One Champ as "farmer" (Kael with lvl 60 should work well) and 3 champs that are only leveled up to serve as food for others. Campaigne stage 12-3 brutal is quite easy to do, gives high exp and silver (sell the shields you get as loot). 

The food champs get no ascension and no equip, they get sacrificed anyway.

5 rank 5 champs needed to bring one champ to rank 6.

4 rank 4 champs needed to bring one champ to rank 5 (20 for 5 champs).

3 rank 3 champs needed to bring one champ to rank 4 (60 for 20 champs).

2 rank 2 champs needed to bring one champ to rank 3 (120 for 60). 

So yes, it's a lot of repetitive grinding, but unless you want to spent all your money on chickens, it's the way to get champs to lvl 60.  

The difference is, that the other player allready had a lvl 60 Kael. With a lvl 50 Kael, you should probably try campaign 12-3 hard instead of brutal until Kael is lvl 60.

On the good side: you don't need to level that much champs, as you have 2 chickens allready. You need only 3 rank 5 champs, 12 rank 4 champs, 36 rank 3 champs, 72 rank 2 champs for your upgrade tour.

It's always been sugested to get your starter champ up to level 60 first as this is probably the one you will use to farm campaign to level up others.

That would mean Kael, or at a push Elhain, Kael over Nekmo as rares are easier to book up their skills.

Get him strong to solo the campaign to level up commons and uncommons to use as food.

Don't feed any of the others you mentioned, just grind away at the campaign with Kael and 3 food champs until you level Kael up to 60.

Get up to where you can solo brutal 12-3 or 12-6 (12-3 gets more silver from selling the artifacts, 12-6 gets more xp) your choice.

Thanks for the replies folks. Regarding leveling up champs, so far I've never farmed xp on them and only used brews; given your answers, I take it that's a big no-no? What should I use brews on?

Brews are a way to speed up levelling. Save them for a champion training event. Actually there is one of these events running for the fusion of the epic champ Huskarul, so it's a good time to use your brews right now.

Sorry to revive the thread 3 days later but what about the 2nd 6* champ? Is Nekmo safe for that or would any others be better?

Not really reviving, just checking back in :)

Nekmo should be a wonderful second 6* once you have your starter/campaign farmer done. Should be a great progression champ. 

Thank you, much appreciated! :)

Actually, while we're here, if it's not too much to ask, could you recommend some items for Nekmo and Kael? I'm currently running Cruel + Lifesteal on Kael and Immortal + Perception + Accuracy on Nekmo (found that suggestion on some random website)

Don't worry about sets. Focus on stats. Lifesteal on Kael is probably necessary - but only for campaign farming. In pretty much all other places, stats are more important.

I'm sorry but I'm not entirely sure what you mean. Which stats should I look for? I noticed that the amount of stars influences the main stat and the rarity influences the amount of additional stats already present (so the rank is more important than the rarity).

What stats you should look for will depend on who you are gearing. I'm gonna copy-paste a response I gave to someone else in another thread, take inspiration from it if you'd like:

For all items - set filter to 5* and 6* items, and rare, epic, and legendary rarities. Then, for:

DPS Champs (such as Kael):

• Top-row items - set substat filter to CRATE% and SPD. If you find you get over 100% CRATE as a result, switch some to CDMG% and SPD instead.
• Bottom-row items - set primary stat filter for gloves to CRATE%, chest to PrimaryStat%, boots to SPD. Set substat filter to CDMG% and SPD for gloves and chest, and to CDMG% and CRATE% on boots.
• Accessories - set primary stat filter for rings to PrimaryStat, amulet to CDMG%, and banner to ACC. set substat filter to PrimaryStat% for ring, ACC for amulet, and SPD for banner.

CC/Debuffer Champs (such as Nekmo):

• Top-row items - set substat filter to [HP% or DEF%] and SPD
• Bottom-row items - set primary stat filter for gloves to [HP% or DEF%], chest to ACC, and boots to SPD. Set substat filter for gloves and chest to [HP% or DEF%] and SPD, and for boots to HP% and DEF%.
• Accessories - set primary stat filter for rings and amulet to [HP or DEF], and for banner to ACC. set substat filter to [HP% or DEF%] for ring, ACC for amulet, and SPD for banner. 

The advice @kramaswamy.kr gave you is great.  I wish someone had told me about that when I started playing.  

Follow up with more questions if you dont understand something or need more advice.  This is a great forum with good advice.