Dynasty Warriors 7

This can easily be acquired by playing through Shu's campaign. During the battle of Changban, you will get to a part where you play as Zhang Fei and must defend a bridge. There's a ton of footsoldiers and power-ups that constantly drop and only takes a matter of minutes to reach 1000 k.o.'s.

Easiest way to do this is just take your time when you are playing a stage and make sure to kill every peon group you run across.

You can also use the time tested method of camping gate captain spawns. Course, they don't have captains and they aren't called that anymore, but they still exist. For the uninitiated, there were gates that lead outside the battlefield, and enemies would enter from these gates, and there were infinite spawns more or less.

However, they don't seem all that plentiful. The only one I've found was during the Yellow Turban stage, the part that has you eventually fighting Zhang Jiao. There is one east of him. It's a fairly slow spawn, though, so you'll want to clear out as much as possible before then. If you are thorough, you could probably get 1k kills before you made it to him, though.

A lot of the later stages have tons of peons all over the place, so you may wish to get 1k kills then.