The trick is having Yukimura and Mistunari in your party. Yukimura's RB attack can kill enemy officers in two or three hits when he's around level 50/60 and Mitsunari's RB attack can clear out a lot of enemies easily and also take off a larger chunk from enemy officers.
Level: Wu Chapter 4-X Battle of Wan Castle. This level is really tiny and the officers come in waves. Also, there are weapons in boxes around the level. Just make sure you open all the boxes in the area. A few will be weapons. I played the level twice and only got a 4th weapon for one character each time, but I did get the weapon at least once so achievement unlocked. Whether you get the weapon or not is pretty much random but the level is so easy, you won't mind running through it again anyway. Another weapon box is dropped by Pang De, I think.
The easiest way to beat this is to stay where the peasants/lady samurai/etc escape through. This way, every time a new wave comes, you can run to each officer separately without getting flooded. Make sure you are at that escape gate when the last wave comes because then you can kill the last girl officer to end the level easier. When she appears, a male officer will appear by the gate. Kill him, let her run over, and then kill her without the other officers of the wave coming near. Super easy.
Again the only tricky part is making sure multiple officers don't swarm you.
Abilities to equip:
Karma - raises luck
Fortitude - raises defense
Conserve - reduces Musou gauge usage. This is important for the RB attacks.
Awakening - strengthens Musou attack
Boost - strengthens charge attack
Refill - refills Musou gauage in battle. Also important for the RB attacks.
Potence - increases attack strength
Another important thing is to make your character's equipped weapon have the right skills to make this level easy. I figured that the following skills on the weapons seemed most helpful.
Fuse Yukimura's weapon with these skills:
Fuse Mistunari's weapon with these skills:
It's important to aim Yukimura's RB attack correctly... it will literally knock out officers so fast that they can't even hit you. I was around 55 with Yukimura and maybe 62 with Mistunari when I first tried this level.
[Thanks to Lucent Beam for this description.]