For me, the backbone of an Inferno army is Nightmares and Fiends (or Overlords). Nightmares live longer if you don't immediately throw them into battle, and a clever move can scare someone. Overlords, thanks to their magic, burn quite well, and Fiends have magic resistance... they're the best unit against mages. And this unit likes to defeat numerous squads, for example, Necropolis units.
The Black Dragon looks better because of its Dragon Breath and Elemental Fury!
I upgraded Marbas's Stone Skin, so it was difficult to defeat him with cavalry... so cavalry is weak because of its lack of thick skin!