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I'll also contribute to this noble cause of bloody carnage between the Dark and Light factions :)
All Dungeon strategies are based on unfair tactics – that's the essence of their race (remember the same Hunters). Therefore, on a large map, you need to max out the Education skill to Mentorship and prepare 4-5 cunning heroes with Hunters and Chaos maxed out. They will significantly thin out the Sylvan army before the main heroes meet. Moreover, in 3.1, the Rain of Arrows no longer hits invisible units. And for the main hero, the above-mentioned tips will be suitable. Especially – Summoning and Fog Cloud.