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1. Imps. Even if the hero isn't a powerful mage, depriving the opponent of mana is useful.
2. Greater Demons, due to their jump ability.
3. Fire Hounds. Similar to Cerberuses, but with fiery breath.
4. 50/50. It seems to me that they are equally effective at each stage of the game.
5. Against neutrals – Nightmares, against heroes – Hellhounds.
6. First, Lords, then Pitfiends.
7. I like them equally. It seems to me that Archdemons are better against neutrals, to pull archers closer, and Archdevils are better against heroes (especially strong ones). Then there is a higher chance of losing a unit, and reinforcements will be very useful. Although, lately, I've been using Archdemons more often.