I don't like the ball behavior. Perhaps the latest build (which I haven't updated to yet) fixes the problem but it has always been buggy.
That being said your problem is that your situation is more complex that just bouncing off a wall. If the player gets really close to the wall you have a potential where the enemy will have to overlap the wall in order to approach the player. You will also run into situation where the enemy will fly past the wall as well.
That's why the custom movement/path movement behavior is better for stuff like this. If you don't choose to go with that then you have to add more complex conditions to make sure the enemy is never overlapping the wall.
This seems to be pretty stable:http://www.box.net/shared/ztv5kcehnj