The creature was as deadly as an arrow. No, it was more deadly than that, for even though it could reach similar speeds to an arrow when it really caught the grip of gravity and sped towards its opponent, it could also correct itself mid-flight, making it a mightily difficult task for the average person to dodge.
Seeing it was the battle for Oliver. Now that he'd seen it, the bird was as good as dead. He cut it straight through. With two dead, their chorus was just ever so slightly duller, enough so that he could just barely begin to hear things.
It was a test of perception of the likes that he'd never dealt with before. When Dominus had spoken of sense in the past, he'd spoken of something akin to instinct. When it came to Oliver's weakness of detecting that which was behind him, he could only detect something that he was focusing on, and even then, it was only if he could hear or otherwise detect its approach with his senses.