WARRIORS OROCHI 3 Ultimate

This solution will be much easier if you have one of the mounts; I used the Red Hare.

Load up Chapter 7: "War Against the Serpent King" on Easy difficulty.

Do all of the objectives and wipe out all of the enemies surrounding the castle until you receive a new objective to "lower the drawbridge in the south for your allies". Don't lower the drawbridge. Move to the middle/northern most part of the map and your allies (Lu Bu and Zhang Liao) will follow you and help you wipe out the endless spawn of soldiers who will rush out. Occasionally the enemies will stop spawning (they did for me); run around the immediate area for a couple of seconds and eventually a new swarm will engage you and your allies.

I kept charging through the mess on the Red Hare, back and forth. To give you an idea of how long this will take you, at 30 minutes left on the clock I had 8006 kills. My 10000 kills came in at 23 minutes remaining.

After that you can lower the drawbridge and then go on to beat Orochi and end the mission. The achievement will pop on the Battle Results screen.

I used a different method. I went into Gauntlet mode and played a 5-Link Stage. Basically all you have to do is score at least 2000 kills per stage (2000 x 5 = 10,000 kills). To me that was a lot easier, quicker, and not as boring as farming one specific level. Once you hit 10,000 kills, the achievement will NOT pop. You must COMPLETE the final stage (5th) to pop the achievement. As you are going through the Results screen, the achievement will pop.

This achievement requires you to defeat 10,000 officers in one level. The only bad thing is that there are hardly any levels to accomplish this. The best one I know of relating back to Warrior Orochi 3 was Susano's Last Stand. This level had insane amounts of enemies and guard captains/base captain spawns without either of those two being there.

Mop up the whole level leaving just Susano left standing and go into a mob spawned area. From there just keep mauling till you reached your target and finish the level. Also if it's anything like the last game, it'll be on chapter 8 and one of the last missions you'll ever play in story mode.

If anyone has another level they did feel free to tell and I'll credit and add it to the solution. I'll keep looking for other solutions other that Susano's Last Stand for the 10k kills.