thank u kramaswamy.k
I farm 12-3 Brutal even late-ish game.
Earlier game I actually recommend farming the boss stage of Chapter 6: Palace of Aravia in campaign, and of the Boss Stage of Chapter 8: Valdemar Straight.
Those both drop the best sets for many earlier/midgame accounts in Speed and LIfesteal, and the boss levels drop Rare items, unlike the regular chapter levels themselves, which only drop Common and Uncommon items.
This way you get XP for leveling food while collecting decent items to get your first dungeon team well equipped. After that, you can start farming Dragon on a much higher level. Dragon can give you Epic and Rare items, but getting a particular set or piece is so much rarer. Can take months of farming to get a specific piece from a specific set with a specific main stat.
I've never heard anyone reccommend doing nightmare for XP. 12/3 is the best XP and silver for your energy. It will use a ton of energy either way if you're getting somebody to 60, there's no way around it.
Nightmare gives about 15% more xp per energy than Brutal does. It's just flat-out better purely in terms of xp per energy.
The main reason people don't is simply because it's not easy to get a champ that can solo it quickly. I use Ignatius, and each run is about a minute and 15 seconds, compared to around 10-15 seconds on brutal with Sinesha.