Thank you Macsen Rufus.
The battle was Cumans-expert XL defending from the Horde at Lesser Khazar. I never finished this campagn, just found the replay recently. It was quite a fun (and long) battle. I used mainly mercs at first, including arquebusiers who stood to the end. My reinforcements were almost exclusively Wallachians, Steppe Heavies and Cuman Heavies, with which I mowed down the Mongol archers and chased the horse archers into killing zones. The heroic Trebuchet crew were very likely mercs, too, and withdrew from the field under their own exhausted feet after expending every shot.
Here's a sequence of shots from the replay showing the Khan getting waxed.

Incoming!
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The shot is short
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Splash damage
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Lucky bounce
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The Khan and some others are left in the Horde's wake.
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And the aftermath of the battle
[IMG]<a href="https://photobucket.com" target="_blank"><img src="https://i231.photobucket.com/albums/ee118/fadraga/postbattle.jpg" border="0" alt="Photo Sharing and Video Hosting at Photobucket"></a>[/IMG]