Add periodically some penetrating oil and let it do its thing over a serious periode of time, use a good fitting screwdriver from the beginning for not ruining the groove of the screw.
If you have no success, maybe an impact screwdriver, some gentle heating, could help.
I agree with both previous comments.FWIW,50%automatic trans fluid and 50% acetone is a very good penetrant.Your grub screw absolutely looks peened,so have a close look at that area and if there is a fair amount of metal displaced(that's the point),try 'carefully' removing it with a Dremel rotary file,as little as possible to clear the obstruction.Just my opinion,but before going to an impact wrench,the tools pictured create much more force than a regular screwdriver and can usually be found on Ebay.(except for the one on the left,machinist made it,my favorite,very hard).After that,yes a well-fitting impact tool,after that doesn't work,center punch it and drill it out or bring it to a machine shop.Also,I've found that much of the Italian metal and hardware is pretty soft and can usually be drilled pretty easy,although I haven't had to mess with a grub screw?
The wedged portion of the ball joint that is holding the screw in place, needs to be "gently" (do not damage the threads) coaxed back a bit, so that the screw can be removed. After the acetone and ATF ****tail, I used a very small pin punch to ease mine back just enough to get the screw loosened, and the same punch, or a small screwdriver to reinstall the wedge once everything was rebuilt. Others may have used different approaches to this, but the key is to be very careful in whatever approach you take as this is definitely a "less-is-more" operation.