quote Rampidzier from GameFAQs

On Samurai Chapter 7 - Battle of Wu Zhang Plains, defeat 1000 enemies. The easiest place to get most of your kills at is inside the central fort of the map. The stronghold here is controlled by Masamune Date and will activate when you defeat Keiji Maeda. As long as Masamune Date is not defeated, you will continue to get enemies coming out of this stronghold.

im struggling with this too.

i used lvl 99 Hanzo with lvl 3 weapon decked out.

i found a peasant base on the left hand side of the map about middle. i dont know if thats the right one. but anyway you kill Keiji first, but dont kill Masamune. then you go over there and wait, and there SHOULD be an unlimited supply of peasants coming out until the point you kill Masamune.

i also did have to run around to find a couple more peasant bases around the map, and thats why i went with Hanzo because he's super fast and he can kill peasants just as easily as the big boys (Tadakatsu and Lu Bu), perhaps easier since his final C3 move just messes up a whole group of peasants all at once.