Depending on where the screw is, you might try an impact driver with the bit modified to fit the slot. Garrett Wade sells a small one I think. I bought one from an auto parts store and filed a bit to fit the holes in the striker disc.
The advantage is the it only turns when you hit it with a hammer, so the force is into the slot or holes. I had tried everything mentioned here, as well as putting a bit into a brace. Nothing budged it until the driver.
I'd be a little timid about using it on exposed screws, but it's actually probably safer than the slot than and other method that would be more likely to slip.
Here are the garret Wade drivers:
https://www.garrettwade.com/pro-screw-loosening-impact-driver-set-gp.html(Be sure to tell Garret Wade to send $10 to Dave if you buy them)
