I've had exactly the same thing happen to me. I was in a long archery duel with the Mongols and he lined up his Heavy Cavalry behind his archers and a heap of Mongol warriers and peasants all crushed in together. It was such an inviting target that i sent my crossbowmen within arror range to rips great chunks out of him. I wish I'd have had some siege weapons handy, even a Balista would have caused shocking casualties - it'd be like Mongol Kebabs on skewers.