'Penetrating' oil isn't Kroil.
Never seen anything like Kroil, beats everything I've ever tried by a long shot.
^ KROIL and than mount the gun in a vise so your hands are free. Use a proper fitting screwdriver bit, tight and full width of the screw. Hold it fully engaged and pressed hard into the slot and use some leverage on it.
This is an example of what I would use...
Need to spend a few dollars on the right tools and wait for delivery? Do it. The alternative may be a striped screw slot, a badly gouged stock and an expensive trip to a gunsmith.