You probably know this , but basically you need:
1- troops that increase the max soul you can collect (aka Necro trait)
2- Ways to collect extra souls. Popular are The Dragon Soul and Valkyrie. Some weapons also give souls, and then your hero can be a Necromamcer (get souls on gem matches)
3- extra bonus (armor and difficulty setting)
You aim at a difficulty with the best bonus you can get while still winning fast.