To ad to Frosty's list, try tightening the stuck fasener, don't try and twist it off, just try and get just a fraction of a degre. Also hitting the fasener, again don't hit it so hard you deform the head or crack the cast iron body. Now, last my screw drivers, especially standard (flat) bladed ones come in a whole lot of sizes, get one that fits the slot well, even if you have to modify the screwdriver. , sometimes you have to clean up the slot, using a healers file or a fine tool
The hacksaw or two. Don't get crazy here. Lastly, a bit of Valve leaping compound might just give the screw driver the extra grip it needs.
It her toos and tricks, if you can find a bit that fits well, a 1/4" air or electric impact might get it loose, but be real carful, that goes for hand held impact drivers as well (used for getting stubern screws out of old car doors)