Avoiding armageddon

[DGDobrev]

Your supposition is correct. Against heroes with armageddon, try to stall. Don't kill their units too fast, let them cast demon portal instead till they're out of mana. When your leadership starts to outweigh theirs (I'm not certain by how much), they usually start to cast it.

If you can't stall (especially on impossible, because Archdemons there "half!" your troops like crazy), try to take their numbers down gradually. I had Samman down to 85 mana on impossible, which was only 2 armageddons - and with paladins + phantom, you can easily recover from these losses.