I heard that Brutal is the best.

When I am not trying to build up silver I stick with just the "normal" mode on campaign.  The odds are just the same of pulling all the champs, if you scroll through the difficulties you will see the offerings do not change for non-artifacts.

So what you need is volume, at 4 energy each.  You can also boost level on some lower champs.

But if you are trying to make silver then one of the other levels would be better. Drop rates are typically 1% or so, so I heard from u-tube commentators.

Warmaiden is not rated that high as a champ in a general sense but very usefull in an arena team with diabolist, spirithost, and a damage dealer (starter champ).  Speed tuning is very important to make it all work right.  I have a set of three teams to use in the tag team arena.

The list of offerings does not change, but it says nothing about odds.

Chances for rare champion drops in campaign are 0.5% in Normal, 1% in Hard, 1.5% in Brutal.

What about Nightmare Campaign?

Well I don't have the confirmed answer to this but i'll just assume it's 2% based on the pattern set by the previous stages. It wouldn't exactly be wise to go farming for rare champs on NM campaign anyway, so I guess it doesn't really matter.