Like a Dragon: Ishin!

This is fairly simple but just takes some time. The game will give you a tutorial on recruiting troopers and going on missions in the battle dungeons, which mostly amounts to running through some tunnels, killing everyone inside, and leaving. Complete all the missions listed under Bandits' Cave to unlock this. There are separate achievements for completing the missions for Bandits' Cave, Bandits' Mine, and Sanada Stronghold. If you wait until New Game+ or even the final chapter they should be relatively easy, although you do find lots of useful crafting materials in them that will help you forge better weapons and armor, and they give a decent amount of money per mission as well.