I use a Snap-On impact driver set to get out things like this. But if it is not handy I have had good luck approximating one but it takes two people. Get a large slotted screwdriver that fits the screw slot as full as possible. You definitely do not want one that is too thin in the blade. Use one that has a square profile in the shank and put an adjustable wrench on it down close to the screw. Put opening pressure on the screw taking great care to keep the blade centered in the slot so it does not "pop out" and damage the screw head
. While you are doing this, have your friend gently tap the top of the screw driver straight down on to the screw head
, i.e. not a glancing blow. A dead blow hammer works best but any medium size will work.
Tap, tap, tap while you try to turn the screw with the wrench on the screwdriver. You need two hands on the screwdriver and the wrench to keep it steady and not strip the slot head. Alternatively, without a helper, try using that same large headed screw driver and tap on it yourself while hand turning with the hand on the screwdriver handle. Or just tap with no turning. This will sometimes work.
I have also had some luck by gently heating
up (with a heat gun) and cooling
down the area around the screw as that can help break the "seal" of the threads. But gently. Good luck.