Hi,
iTween could be a good thing here, and as you adjust the sizes, you simply adjust the path. That's the most straight forward method. When you want your ennemy to start doing something else, simply disable that itween path and feature and have your ennemy do something else.
Raycast is possible, but only dive into this if you know you will have a lot of possible variants to this set up ( go around spheres or actually any walls). this will work, but requires a higher level of understand about Unity,PlayMaker and algorythm to do this properly.
Bye,
Jean