The Onis could apparently shoot some kind of laser from their eyes, which caught both Fai and Kurogane by surprise, they didn't know the Onis had those abilities. So since Fai wasn't expecting it he couldn't dodge it in time and didn't have the weapons to defend himself against it, of course he could have used magic to protect himself but chose not to.
And Kuro's sword broke because he used a special technique to end the fight fast for Fai's sake, since Fai was too injured to walk, let alone fight the Onis.
