I've found it quite interesting that people do soul farming. I've never been inclined to do this. When I was super early-game I would farm for treasure maps (this was pre-explore), but now I have almost 1000 that I'm not using, so.
To get souls I would just disenchant extra mythics. I know for some people that's not something they like, or do, but that's been my method. Tribute/Challenges/and Disenchant. Way easier and nice now they brought back the mass disenchant button. I would spend an hour sometimes disenchanting everyone by hand before that button came back.
I'm now level 1050+ and have 13 more troops to level, 9 of which are still level 1. But I'm not rushed or worried about it and frankly, never dedicated a team to farming. Nor did I really build a team with that skew in mind. I've just done all my challenges, gotten souls from tasks, and disenchanted.
EDIT: I should add that I'm one of those "unpopular" guys who would disenchant extra mythic troops down to 1 (not 4), including all base legendaries and mythics. Those are nice souls, so I've done that a lot. In the unlikely case that devs add another tier then yeah, I might be worse off than others who won't/dont, but I'm willing to take that chance.
There are some troops that I know I want to have 2 or more copies of to field specific teams and I save those, but everyone else goes.