Dynasty Warriors 7

In conquest mode, select the mission "Battle for Supremacy." This is a 5 red star battle next to Diao Chan's legendary missions. The mission itself pits up up against basically every officer in the game with a contingent of soldiers.

Killing every soldier and officer on this map, I was able to get away with 4037 total kills in fairly short order without stationary farming (there are no enemies spawning from gates on this one).

Keep in mind, that being 5 red star, you're going to need a decently levelled officer with good weapons and seals

This is easily done in the Wei Story battle, Battle of Wan Castle. You'll be playing as Dian Wei, escorting Cao Cao out of the castle.



Once you reach the portion of the map where Dian Wei and Cao Cao are in that picture, stop, save and fight. So long as you do not defeat the troops in the south, with the flame statues and the ballista, enemies will continue to respawn here at a fast pace. Keep picking up axe powerups for double attack and the kills will just come to you. The achievement pops immediately after the 3,000th KO, so if Cao Cao doesn't have enough health to finish the level, reload the save and continue on as normal. (Thanks for pointing that out, Tohru Hybrid!)

Given the amount of enemies and the amount of powerups dropped, this is also a good place to grind for Gourmand.

During the Story Mode of Wei, you have another battle where you can unlock this cheevo. It's the Yangping gate Battle and you are playing Xhiaou Yuan. In this battle, all the troops respawn, due to the power of the Zhang Lu.
What I did is, after clenaing up a litlle the map for the officers, i stand myself just in front of Zhang Lu castle (which is at tne North of the map), all alone with Zhang He and kill all the ennemies who respawn almost immediatly and that, forever (almost).
I am not sure but you can also target the 100.000 KO achievement here if you want (but only if your are very patient because it you will take a very long time to do so).
One last thing: if you clear the map all the officers except the general, you dont have to worry about Cao cao been killed. In this battle, he stays in his headquarters castle all along.

Easily obtainable on the Conquest level "Battle for Supremacy" on Beginner Difficulty. This stage is directly to the left of the Jiao Province, at the bottom of the Conquest map.
The map will start off with the requirement to face the Wei, Wu, and Shu forces. Be sure to defeat each and every enemy before you decide to attack the defending leader of each group (Liu Bei for Shu, Cao Cao for Wei, and Sun Jian for Wei); at this point you should be around the 2,840 kill mark if you've done it perfectly.
Afterwards, Sima Yi will approach (Jin forces), so just finish off with them in the same manner; this should get you easily up to 3,750 kills.
After defeating Sima Yi, Lu Bu will spawn; defeat him to not only complete the stage, but to also unlock Red Hare.

EDIT:
Also, if you're going to vote negatively, please at least be productive by providing some sort of input. Otherwise you're just wasting everyone's time, my own especially, as I've actually used this solution to get the achievement myself.

I did this on Wei story mode on the"Battle of Yangping Gate". The officer you must defeat to win the match can heal the soldiers. There are about 3-4 spots where you can kill the soldiers and they will continue to be healed so you can keep killing them. The best spot is right before you enter the gates of the final officer. There are three large groups of soldiers that spawn and get healed. The other spots on the map only have one group. It takes some time so be patient. I can post a video if it is requested.

This is a good video showing how to get the achievement during the Wei storyline. The battle is Wan Castle Part One.

*This is not my video, all credit goes to DarthRavan2012 on YouTube*