Rubber hammers are your friend. After you make sure the gun is unloaded and the magazine is out.
Once you get it off, get a bottle of sewing machine oil, slather it on the rails, and install the stripped slide on the frame. Work it back and forth a few hundred times, keeping the rails wet. When it starts to move easier (no less that 400 cycles), get a little silver polishing compound and mix a half-and-half portion of silver polish and sewing machine oil. Put it on the rails and cycle the slide around 400 cycles. Finally, take a teaspoon of the same oil, and add a hefty pinch of
Barkeeper's Friend, mix it well, and go through the same procedure. After cleaning it up, the slide should not move when tilted at 30-35 degrees, but when tilted past 45 degrees it should start to move.