Every time you move to a square - one step at a time - he will move two steps. You must use the fences to keep him at bay.
These puzzles are difficult. Of the 100 levels, I've done about 10.
About 5 years ago Jason Fieldman came out with Theseus, which is basically the same thing. Though it seems more polished in terms of the graphics. I also had trouble with that one.