On Chapter 2-1 you will eventually come to a concrete bridge that has lost a section, meaning you'll have to jump across. First, take away Sheva's weapons so she doesn't interfere. Then jump the bridge and walk forward slowly until a cutscene shows with a suicidal truck. Shoot the red barrels on the sides of the bridge to turn this beast over then run up the the corner of the truck. At the other end of the bridge you'll notice several majini with bowguns. Again, walk forward slowly until they start shooting. Even if Sheva has weapons, she shouldn't attack but it's better to be safe than sorry. Raise your knife and just start swinging. Yes, you will take damage, yes this may take a while, and yes this does involve quite a bit of luck but with multiple arrows, the distance between hostile and you, and the fact that everyone's stationary makes this the easiest way to get it.