you can use a dremel or file to make a small straight groove in the middle of the screw head which will help with markus's suggestion of using a flathead screwdriver. it takes a bit of patience and some trial and error, but eventually you will be able to fit the flathead and remove the screw.
+
+
=== update ===
+
+
The answer I provided is pretty funny to me now... I had to do a teardown on mine and I stripped the EXACT same screw on the EXACT same model :)
+
+
I ended up using a small pair of grooved grooved pliers to turn the screw from the outside. Maybe this needs to be noted in the battery removal guide, seems like they extra-loctited that screw or something.
you can use a dremel or file to make a small straight groove in the middle of the screw head which will help with markus's suggestion of using a flathead screwdriver. it takes a bit of patience and some trial and error, but eventually you will be able to fit the flathead and remove the screw.