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I haven't heard of a manual, and I don't see the point in it. Since all in-game tactics are imperfect (in any of the games), and unit compositions are also not perfect, you have to figure out for yourself how to defeat a particular unit. If you're interested in the lore, there are books (if they are published, they are approved and are part of the lore), and as for tricks, it's about character abilities, magic, and upgrading the main hero.