I would add to the list Season 2: 3-4 & 3-8 (normal level).
For which color?
I used those levels for my last fire chest, so maybe… blue?
Try S2 3-4, it gives 13-14 kills but it takes 6 flags.
I play there for better rewards and not recruits but it gives the most red in S2 I think.
Orihalcon, drag bones and meteor.
Yes Season 1 is very poor for red monsters and Atlantída 3-4 is the best choice and more easy for weaker player.
Farming red monsters
From S3, 6-2 seems to be reasonable
Yeah, that’s what I usually use. About 14 monsters for 6 energy
You are so right, I wasted my energy on 20-4, 4-1 is definitely the best level for farming reds.
try the season 3 muspelheim special stages. i think one of them gives 15 enemies for 9 WE.
If you are just farming to fill an elemental chest, why not go the low cost route as determined by @BarryWuzHere: