Mark of the Ninja: Remastered

I did this pretty easily in stage 3 during NG+. You reach a point which is pretty large open area with enemies on catwalks. You can use darts to make enemies get closer to the border of the platforms and then the terror dart can be used to hit the enemy and will step back a few steps and fall to his death.

Make sure to shoot the terror dart from the opposite direction of the border of the platform.