It's a good thing then that Killingham is no longer holding back.
I've suspected Zoro keeps his left eye shut because he's been training it for a specific purpose, like Deidara from Naruto.
Killingham could very possible conjure illusions he is able to see through with that eye.
I'd like it if he's got like an "Eye of Asura" or something along those lines that allows Zoro to use FS or some other ocular technique to enhance his swordsmanship.