Soul farming requires a troop that garners souls upon casting their spell, Valkyrie is a personal favorite. 
And one or more troops that have the necromancy trait which increases maximum souls gained per battle.
A very old, standard soul farm team is:
(Blue Tank/Dmg Dealer)
Valkyrie
Alchemist
Banshee
This allows you to control the board and continue to fill banshee and Valk who keep garnering souls. Valk also fills the Blue Dmg Dealer, who ultimately wins you the game. Behemoth is a favorite if in the first slot, I also like Rowanne in the last slot. But you can play around with that at your leisure.
You can also check out this thread and might find some good ideas there: