Another vote for the soldering iron - and tapping it with a hammer too.
Heat it up, jar it loose.
And of course use a flat-ground screwdriver that fits the slot exactly.
Sometimes I use Kroil, letting it soak in overnight.
I used to have a security outfit bring me a shoe-box full of .38 snubbies to look over once a year, whether they needed it or not. - Every once in a while I would run across a stubborn screw like that.
Good luck, and let us know how it goes.