I am a technician, and here’s another hint that I am constantly reminding other techs about: When you encounter a screw that is difficult/impossible to remove, the moment you begin to realize that your efforts appear to be doing little more than stripping said screw, STOP.
…then instead try to tighten it a little more. Repeated attempts to unscrew it may have started to ruin the anticlockwise surfaces, but the clockwise surfaces may still be solid. Tightening the screw even a few minutes of arc (especially since the downward pressure you are applying will actually assist you in this) may be enough to break it loose enough to reenable unscrewing it.
–Patrick
