When I have this problem with a torx screw I use a small flat screwdriver that can enter between two notches and apply a little force to unscrew the $%#@$%? screw. Maybe you can try this trick. If your MBA is no more covered by the Apple warranty you can also use a Dremel with a small rotating blade to make a notch and then use a flat screwdriver to take off the screw.
http://www.dremel.com/en-us/Tools/Pages/default.aspx