Arching Blades can be quite hard to hit, especially when trying to knock down enemy champions, due to its unconventional path, very small knock down area, and somewhat slow speed. Using it from a stealth pad would make it slightly easier.
If Arching Blades knocks down a nearby drone instead of the champion you are aiming for, consider blinking to it with Unnatural Reflexes instead of the champion. Blinking to an enemy champion grants Bloodlust, you gain up to 20% damage on basic attacks when attacking from behind and 25 move speed. On the other hand, any knocked down enemy will grant doubled bonus attack speed (up an additional 35% bonus attack speed) and retains your damage reduction, which can be more useful especially when your target intends to fight instead of flee. The latter is also not gated by the Hunger available, although blinking to a champion will allow you to immediately and automatically hit them with a basic attack.
Unnatural Reflexes does not do any damage on its own. It can be used on a friendly drone. When using Unnatural Reflexes, Nightmare Robin always appears behind his target, no matter which direction they are facing. This makes it even more useful for dodging some skillshots while fighting enemy champions, such as Gaslight Batman's Bat Scream.
Nightmare Robin gains Hunger and the Attack Speed boost even if he uses it on a friendly drone, allowing him to take down turrets faster.
Since Unnatural Reflexes can be used on a friendly target, it can be used as an escape. Nightmare Robin can even blink to Regen Pads and Surveillance Cameras. The latter is particularly useful as it can be deployed anywhere, especially when using Unnatural Reflexes to blink through walls. For the purpose of Unnatural Reflexes, despite the camera's spinning animation, it is always placed facing away from the champion (i.e. if Nightmare Robin puts down a camera and immediately blinks to it, he will appear on the near side of the camera). Putting down a camera just outside the base allows him to return to lane slightly faster and gain some Hunger.
While Arching Blades and Impale do not need a target, they need to hit a target to generate Hunger.
While it is still a basic attack reset, Impale will not deal bonus damage to turrets, but it will not be consumed while attacking turrets either, despite that every basic attack for its duration will play Impale's sound effect and animation on the turret.