I got it when AI sieged me during winter season and lost. I think I had my Damiyo defending there. I have DLC packs and I was playing the Rice of Samurai campaign.

I think the rarity of this archivement is because AI doesnt siege during winter seasons that often, players tend to keep their damiyo out of harms way or in attack and it needs Rise of Samurai expancion.

I cant remember the exacts but I was shorthanded there; I pretty much took the castle and then noticed the full stack right next to it. I kind off escaped to the castle and AI followed there to flush me & my damiyo out. It was relatively early ingame so I had the advantage of cavarly with the damiyo and another generals boduguards and advantage of melee with few sword attentants. Rice of the Samurai archers are terrible so I ignored them and whent for their leader; Got him down with 1:2 numbers + charge. That resulted the morale shock to the AI army and front line that faced the few Sword Attentants of mine, turned tails and caused mass rout in the blob coming up the hill. That was it; Lucky in away or rather; Result of AI failing to understand that enemy generals are anti general cavarly and the most important thing in pesant army is the general.