What matters is the Souls gained per hour (or min) not that you can maximize a single match with 3 necro troops. Sure you can get more in a single match, but in the same time frame you do one match, I can easily do 2. Because of that I have significantly more souls collected:
the two best teams:
fastest team out there that still gives some souls.
and my preferred method (on WL2)
This team will kill a match every 1.5 min on average and net 340 souls a match for me. In other words over 200 souls a min, which is way way faster than the suggested team. That team is lucky to average 2.5 min a match.