hI must be doing this wrong. Here is what I am using:
Wight/Ultra-rare(Blue)/Level 17, Traits 1+2
-Steals life to heal and damage, creates gems AND Gains 10 souls per use. (More than Valkyrie)
2X Warlock/Rare(Green)/Level 15, Traits 1-3
-Necromancy and Human, extra Magic for all 3 blue allies, deals damage and removes the purple gems that Wight creates.
Acolyte/Ultra-rare(Blue)/Level 17, Traits 1-3
-Necromancy and Human filler currently. Bonus souls, removed two skill points from enemies, and can create mana for Warlocks if no viable moves otherwise.
Soul meter goes to 100. If I fill that, I got about 320 souls. I use the flag that matches the Wight. Difficulty is only set to hard
If I fight spiders that keep spawning, or kill everything with the Wight, I can get 600 souls. Maybe not as fast, but as easy.
Proof of concept:
Edit: This fight took me 1:54 on 3X speed.