The worst smackdown I ever had in R:TW was when I was playing as carthagenians and I was besieging an egyptian city, I think Shiwa, and the enemy was outnumbered 2:5. I have a twelve stack army of well rounded early carthaginians (skirmishers, slingers, militias, iberian infantry, round shield cavarly even 1 merc eles). The egyptians was defending a stone wall with few peasants, desert infantry and slingers and 1 skirmisher unit.

I sapped the wall and charged my half army with eles in front and was waiting to win an easy victory. The skirmishers managed to rout my merc eles and as eles was running amok killed many of my soldiers who was following in city's narrow roads. Meanwhile the slingers volleyed my men and managed to rout many of them!

I charged my rest of the army and hunted out the enemy skirmishers and slingers with my cavarly and sended the iberian infantry to take the plaza.

My cavarly was slowly decimated by slingers and finally routed while it was fighting with desert infantry!

I had already lost 3/4 of my army but I was holding the Siwa's plaza with my remaining 3 iberian infantry units. The egyptians charged at me the remains of their desert infantry, but I was easily wiped them out.
finally the enemy with 2 slingers units managed to scare my iberians and make them rout!

Result: I killed about 550 of them and lost 800+