And I usually completed this scenario with one hero for any Castle, the others only helped slightly, which is why the result was 38 days. Of course, you need to try to complete it within 3 weeks.
To take Dragon Graveyard on day 3-3, or perhaps 2-4, you probably need about 16 upgraded Swordsmen and about 20 Archers, and even then, there are doubts.
For the Black Dragon, you need at least 3 hits from the Swordsmen, and considering that as soon as the Archers run out, the Swordsmen will start to be depleted.
The most annoying and common thing is that you draw up a plan - this one goes here, captures that, this one goes there, captures this... I can build such logical chains more easily than chains of heroes. And then one link in the chain breaks, and everything falls apart - it's like a Repin painting. We capture the Warlock's Castle on day 7, and suddenly there are no Minotaurs. And we have a meager group of Knight's troops, a weekly increase of 8 Minotaurs, but no Dragons from the Warlock. And here, instead of a blitzkrieg in 20 days, it takes almost 40, and we have to wait a week in the Warlock's Castle. Because even with Medusas, it won't be enough for an army of, say, 10 Trolls, 12 Bog Lords, 100 Goblins, and 15 Wolves. And there's still an army and a lot of Orc Chiefs in the Castle, and the wall shoots accurately.