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Wait until you have 10,000 skeletons... I managed to get a maximum of 5,000, but my opponent already had 25 Death Knights:).
And those griffins were neutral – therefore, without artifacts. Griffins have this feature: they fly up, and you wait for the beast to come down. There were no other creatures in that battle besides the griffins. Imagine the scene: an army of one color stands on the right, and there's nothing else:)! What do you do?
Move your archers back one square, and that's it!