P.S. It's not difficult to level up Black Dragons, the hard part is building them. In one of the games, by the end of the second week, I had five of them (8/6) and they were for the Barbarians. In the third week, they were already leveled up.
P.S.: There's no need to even talk about Inferno and experience. When four Imps turn into one Demon, it's just crazy :D
Added 2 minutes later
And how is it "not difficult to level up"? 140,000 experience points don't just appear on the road.