In theory, you don’t need your 4th troop to do anything, just sit there.
Thrall/Marilith → Zuul’Goth → Thrall/Marilith → Zuul’Goth etc. for the Green/Purple colors
The only thing that interrupts that is excessive unexpected skull hits, for any reason.
With delves now giving enchant all and explode 2 gems per ally at the start of battle from, hoards, mana block isn’t as major of an issue provided you can keep Thrall/Marilith unblocked. Even Essence of Evil isn’t that important.
Huanglong/Marilith, Glaycion/Marilith, Krystenax/Thrall etc.
If you want to go cheap, Eternal Sentry/Marilith is solid.
Something like
Eternal Sentry
Marilith
Rope Dart/Essence of Evil (hero class with Root trap preferred, Orbweaver with Rising Shadow is big)
Zuul’Goth
Banner of Sin
will feel pretty good