User blog comment:IamJakuhoRaikoben/Chapter 384: Attack of the Stars, Review./@comment-78.145.24.241-20140516081841/@comment-7418318-20140516084515

He might have intentionally cut only the places were no FT mages are merged into the walls.

That could also explain why he didn't slice Jackal apart, if the Demon is either standing so close to Lucy that she'd be caught in the attack or if he's standing on top of an absorbed FT member. The CSK certainly seems too big to just walk (or fly) through the corridors to reach Lucy's position and just stab Jackal with his sword.