How to remove a screw from electrical box?
The best way to get it out is muscle. Not much else.  Use a vice grip type plier to get it out.  If it breaks off go to the other side.   Some people say tighten it up into a drill and use the drill to get it out.  When you get it out then re-tap the screw hole.  In the U.S. electrical boxes are 6/32 and most DIY's and electrical wholesalers sell taps for this reason.  Good Luck.