Mount & Blade II: Bannerlord

I did a test early tonight, apparently you gotta kill 10000 by yourself. The easy way would be siege other castle or settlement. Crack at least one wall, equip 1 bow and 3 arrows. Attack, set all your troops to loose and let them wait outside of the cracked wall. Then you ride your horse near the wall and start shoot everyone inside. When you ran out of ammo you could either retreat or spring inside the wall and find yourself an arrow bucket.