What Schierke is talking about specifically in the scene you're referring to is his giant "cloud mass" form, which is something achieved through a combination of magic (astral projection) and his apostle power. That's why she says so. That's also why, after Guts strikes his weak point, that cloud form dissipates and we see him far away in his quarters screaming in pain.
Ganishka was pretty unique, both because of his unusual apostle form and because he did use it alongside some rudimentary magic thanks to Daiba's knowledge and skills. That's how they could create and control the Daka and Pishacha.