DYNASTY WARRIORS 9 Empires

I seem to consistently take under 10% casualties when putting down Vagabond units.

Once your kingdom reaches a certain size, Vagabond units will occasionally form in your territory and attempt to seize a province from you. These units are just scraped together from unaffiliated officers in the area, and usually have about 5 low-tier officers in them.

When you receive the notification that a Vagabond unit has formed, immediately move some forces onto its province. When the battle starts, just smash through to their main base, capture the surrounding bases to spawn their leader, and take him out. Don't even bother with a secret plan - the longer the battle goes on, the more casualties you will take. If you can finish them in 3-4 minutes, you will probably get this Achievement.

I have also managed to meet the requirements while fighting conventional forces, so you don't really need to rely on Vagabond units if you don't want to. But the key is always speed.