Or you can try some epoxy. Put a small amount onto the screw head, do not let the epoxy get into contact with the pickguard. Let it stand for a while. When's it's become semi hardened (not completely set, yet), use a philip screw driver to imprint the + on the epoxy and then let it completely set. Once completely set, you can try to remove the screw with the screw driver.
The other way is to drill thru the screw head (just enuff so that the head would come off and leave the shaft protruding out of the wood, remove the pickguard and use a plier to turn the remaining screw out of the wood. Use a HSS drill bit the diameter of the screw shaft.