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It's not a given that we'll be playing against Chaos – why does it have to be Black Dragons specifically? :confused:
Besides, if we're playing against a human opponent, they won't act predictably. If we position Devils to be targeted by Black Dragons (and by the way, why did you suggest simply positioning them instead of attacking enemy heroes – wouldn't the Black Dragons also have to divert their attention to do that?), it's not a given that the Black Dragons will immediately attack them. The opponent might focus on the Devils with a powerful hero, and send the Dragons forward – they'll reach the edge of the map by the second turn, no matter how the enemy heroes try to hide. And for the Devils to be a significant enough threat that a powerful hero can't easily handle them, you need more than ten of them. But you don't always manage to get ten Devils alive)))
As for Black Dragons and skeletons – I disagree that they are a death knell for heroes. When I play as the Undead faction, I always use an aggressive strategy. Forward and only forward! I try to reach the enemy as quickly as possible and attack first, even when my units are only level 4. Therefore, two enemy Black Dragons or four Phoenixes don't scare me – they won't defeat my Grand Master (or even Expert) hero in the first turn.