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A familiar problem, I managed to overcome it after a million attempts, after reading a step-by-step walkthrough. You need to quickly capture the lower castle, after capturing three mines – gold, with Ents, ore, and wood – and then quickly rush to the upper left castle, waiting for the enemy hero to leave. You need to rush through the stone elementals, as it's easier, because the phoenixes deal a lot of damage. I managed to survive them with a couple of dogs and a pitiful imp. Then, go around the lake and wait for Gilraen to come out of the castle, and then quickly capture him. Also, don't be stingy with additional heroes; I bought about three, and they promptly delivered my troops.