Interesting, I guess. But if it's a pendulum, then she'll have to sway back and forth, so you only really need to put your sword right in the middle of the motion to cut her up nicely.

If it's supposed to look like an illusion of her lying on the ground, then it'll have to be at a speed comparable to Mira's phantom flash - only that the movement is even more predictable since it has clear start and end points.

I suppose it's a technique that relies on surprise though. Once you think it through, it's not so hard to get its weakness. Roxanne, on the other hand.. Damn she's evil.