Any unit or shaman can walk through narrow passages (where they normally could not). All you have to do is selecting the unit, using the mouse move click, and press "d". This can take time, but will make the unit move to a point, in a narrow passage. The shaman can do this trick easier, by casting a spell out of range, in the direction of that narrow path.

Watch this video in order to understand the concept of walking through narrow paths.