Devolik
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I looked at the screenshot. I think it's quite possible to defeat him with your army. The AI is slow; it doesn't always leave the gates while it still has archers. And if it does leave, it does so one stack at a time. You have a decent army. I didn't have one like that in any of the missions. The maximum number of archers I ever recruited was 1000 for a quest. Usually, it all ends much sooner. I'm playing on normal difficulty. Maybe you should reconsider your hero's build and your battle strategy. If you're really losing, I suggest watching how others play and taking something from their strategies for yourself.