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I played Broken Alliance on the difficult level against 5 strong AIs two more times. In the first scenario, I started by clearing out the tough neutrals at gate 16 with 16 Spectres plus one Skeleton Warrior (1st screenshot). In the second scenario, I started by clearing out the tough neutrals at gate 8 with 8 Spectres plus one Skeleton Warrior, and this is what I got (2nd screenshot). In the second scenario, I upgraded Stone Skin and Retaliation 1, 2. Stone Skin is a great ability; Spectres, combined with Regeneration and Stone Skin, can withstand the attacks of tough neutrals with minimal damage. With 8 Spectres, I managed to defeat 56 Dogs and 50 Fanatics. 14 Spectres easily defeated 70 Pearl Priests (even though they deal magic damage); at first, I was afraid to fight them, but then I easily won, although my hero was already quite powerful (Luna). There were no economy heroes.
I upgraded the heroes as follows: Regeneration, Education, Stone Skin, Retaliation 1, 2, Logistics, Secret Keeper 1, 2, Guardian 1. I'm not sure about Retaliation 1, but I didn't really understand Retaliation 2, as I only got the chance for an extra attack once or twice during the entire game. I had the Necromancer's Amulet, and that's a great item; I hadn't really noticed it before, but with it, you quickly gain Necromancy points.
I'm attaching screenshots of two different games. In both scenarios, it took the same amount of time: 13 or 14 days. On the way, I got distracted by battles with neutrals for resources (but they aren't really needed for this tactic). If I hadn't, I could have played faster.
SUMMARY. It's possible to play 2 or 3 days faster. But that's not as interesting.