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This happens if no one has been given a DD for a long time. At this point, it's about persistence. One or two side heroes get small armies from secondary Castles and fight over resources. Pandoras and Dragon Utopias are very scattered; there is practically no "over-farming" as such. Magic Towers are built up to search for Flight or DD. In one game, someone built three of them up to level 5. Dragon Utopias are also searched for—they should be on this template, especially in the underworld. And expert Air or Earth vision wouldn't hurt.
If the opponent is Tower—we look where there is snow; Swamp—then we look at the swamp, and so on.
Sometimes it helps.