2500 Gondorians (mostly militia) commanded by my heir Arathorn (not the real Arathorn, just a guy that married into my family) were besieging Minas Morgul with 2500 Orcs inside (commanded by a Nazgul). 5000 Orcs show up to reinforce them, so I form up on a hill, and let the 5000 come to me -- I sortie from the hill with cavalry as that large army is defeated, and chase them a couple hundred metres until I run into the 2500 w/ Nazgul. My cav fall back, my archers run out of arrows, so I charge with my 15 battalions of infantry into a roughly equivalently-sized Mordor force, and after a grinding melee win with 1300 men left. I figure 'yay, I get Minas Morgul now', but another stack of 2800 shows up -- after beating that (with minimal casualties) I manage to finally take the city. Killing > 10 000 orcs with one army in the same turn = ftw.
15 Screens from the first battle, followed by the Battle Results screens from both battles. Playing on vh/vh, no changes (other than TATW, obviously).