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IMHO:
1) The Dungeon is not very suitable for rushing because, at the beginning, it lacks strong spells and has a fragile army (few Hydras). It's best to attack with the Dark Elves in the early second week... when the guild is already decent and the troops are durable. But it's best to ask those who play Dungeon well about this.
2) Regarding Sylvan, I don't entirely agree with you either. If you build Druids at the end of the first week, you can start rushing as soon as you buy the following units: 20 Sprites, 20 Dancers, 14 Hunters, 8 Druids (divided into 2 groups of 4 or 4 groups of 2). However, it's not worth prolonging the game with the Wood Elves; Heaven and Necropolis benefit much more from this. The Dungeon builds a strong army and upgrades its guild to level 5. The Academy creates good artifacts; perhaps only Inferno doesn't benefit from this. In terms of unit production, Sylvan only wins against the Dungeon... so in the late game, it's definitely not Faidaen's kin that dominate.
P.S. The game's tactics largely depend on the map and the player. For example, I don't really like quick battles and don't see the point in finishing the game in the 2nd or 3rd week.