The key trick is in how you make the slayer navigate the edges of the chamber.
×
Here's an illustration of the basic idea.
Imagine the slayer is moving down the right column. If all the force arrows are facing right, the slayer will be forced to move to 1. Now make the arrows face left, and the slayer will move to 2. Make the arrows face right again, and the slayer is forced to traverse that column. You'll have to do this both when setting up the wisp and when the slayer actually reaches you.
The reason for this setup is that it forces the slayer to traverse an entire column, but avoids giving him shortcuts that can let him jump ahead. It's been a while since I did this room, so there might be a detail or two I'm forgetting, but this is the basic idea you want to follow.
