It’s a bit tough to create the really good soul generation teams early on when your resources are limited. For example The Dragon Soul and Pharos-Ra are very popular but are a legendary and mythic respectively. Early necromancy troops like Valkyrie and Warlock are a bit slow unless you add some units to quickly clear the enemies, but each non-soul increasing troops lowers your soul count per battle.
I use The Dragon Soul, Pharos-Ra, Hero w/ Imperial Jewel (whatever class I want to level), and Umberwolf. Gets around 200 souls a run depending on how high I get the soul count. It’s not the fastest team, but it can reliably clear dungeons and most Adventure Board battles so I use it to pick up some extra souls while clearing dailies.
Dragon Soul kills everything fast and generates souls. Pharos-Ra has the best soul trait and top off the soul count. Hero is there for class XP and to soak up extra mana. Umberwolf just sits there for the trait that creates a darkstorm every turn for Dragon Soul to loop on.
You can swap the hero for another necromancy troop if you want more souls at the expense of hero XP, and Umberwolf for a necromancy troop if you want more souls in exchange for potentially slower battles.