Boss: Oboro and Ninja
Attacks: Shadow Self, High Blade, Impulse, X Cutter
Attacks: Impulse, High Blade, Hide Blade, Spoilsport
This can be an easy fight. Make sure you have enough potions to revive your people and enough to heal them (Fully heal HP) because they will die a lot. Next, work on Oboro and the Ninjas (also his clones but leave one alone), then just work on Oboro and he will die, rewarding you the achievement.