I was finally able to do this today after many many attempts at it....Basically I didn't do anything special...I avoided killing as many officers as possible and just kept on the regular enemies...I worked from the middle to get rid of Keiji and went around the whole map collecting kills around the bases...it takes a little time to do it this way...I found that keeping Nobunaga in the party made it a little easier...he had a bad habit of running into battle and getting himself killed eventually...after I racked up near 900 I killed Masamune and worked my way back to Da Ji...to get the ambush troops near Da Ji you have to kill Masamune first...I ended up with over 1200 KOs by the time I finished the session.
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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.
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