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It all comes down to speed and gold.
Take all the chests for gold, and buy all the heroes of a specific class (warlocks are best), give the starting army to the "leader," and by the beginning of the second week, he should have already captured a couple of castles to buy reinforcements. The rest of the units consume resources and occupy mines.
Then it's just a matter of technique.
And the larger the army, the higher the chance of finding some friendly neutrals.