Another trick is to Mig weld a M8 nut on the screw, than you can use a spanner instead of a plier for more control.

First build up the screw a bit then put the nut on top and fill up the hole.

The heat useally does the trick, sometimes a few tries but never let me down.

Works also very good with broken studs, even when below the surface.

Good luck !

And if you succeed you can grind the nut off, turn it flat, saw the groove back and use them again, 🙂🙂