First of all, disconnect the battery from the motherboard. Dropping a metal object onto the motherboard while connected to the battery could cause some major damage.
Get a small metal drill bit, electrical tape, and non-conductive anti-static material. Use the material to cover the Macbook's internal components and hold it in place with electrical tape. Drill a tiny hole in the centre of the screw. You can keep enlarging the hole until you are able to grip the screw with tiny pliers and twist it out. Alternatively, you could use the tiny drill but to cut a pattern in the screw top that will allow you to insert a screwdriver, such as a Philips or slotted, to loosen the screw.
Another alternative would be to use the tiny drill bit to drill into the plastic ring under the screw. The plastic under the screw is part of the battery, not the Macbook.
