Most of the time players in RAID use a campaign farmer to level up champions, which means that 25% of the experience is always lost as the farmer champion takes 1 slot out of the 4.
It is possible to plan ahead and stop losing this experience in some cases.
That means when you are leveling up a few champions, who you plan to keep, to level 30, then to level 40, 50, 60 - it is possible already from around 4 stars and around level 20-30 to make them independent using +8 to +12 equipment.
And only 2-3 out of the 4 champions are required.
I give you an example from my own account: here I removed my campaign farmer and am able to run 12-3 Hard with just +8 and +12 equipment and 1 additional food champion.
Here, the food champion and Renegade die quicker and sometimes Ghrush the Mangler dies too, but ultimately it is a successful run and that's all that counts. See gears below.
And here are the gears on them; as I said it's all just +8 and +12 which is enough to get the job done.
Broadmaw:
Ghrush:
Renegade:
Conclusion
Using this method is significantly more efficient and results in more rewards of all types than doing it with the campaign farmer.
If you are leveling up someone strong like a Fusion champion you can even do it with only 1 champion and 3 food champions. I was able to do this with Hurndig and will do it again with Versulf.
If you have lots of silver and can boost the equipment to +16 then also you can carry 2-3 food instead of 1.
And otherwise, as mentioned just 2-4 champions are needed with +8 and +12 equipment, ideally one of them more bulky. When you are planning which group to level up you can also plan that one of them will have a shield or resurrect or heal which would be your bulky / tank champion.
Finally the "most difficult" is when they are 3-4 star and it gets a whole lot easier as they level up and rank up because they are constantly getting more stats and therefore getting stronger and stronger.