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Well, you've found something to argue about! How to play as the Elven faction? At different stages of the game, and with different heroes, it's done differently, or rather:
Battles with neutral creatures:
1) At the beginning levels - defensive play (we protect the archers).
2) At the middle levels - at certain moments, we attack the attacking creatures.
Battles with the opposing player/computer:
1) At the beginning levels - we protect the archers until the enemy army approaches (it rarely has time to approach).
2) At the middle levels - we attack the enemy with strong creatures, and defend less against those attacking the archers.