Troublesome screw, could use some advice

[boltman223]

If you have a drillpress or mill.... I was gonna say if adding some heat doesn't work try a left hand drill bit, but it sounds like the screw could have left hand threads. So, whatever it is, a drill bit that is opposite the threads should take it right out. I'd add some heat to add a little insurance that it will come out

 

[MislMan]

Small bladed chisel to clean up and define the slot. Maybe some heat from a heat gun or blow-drier to saturate the area and breakdown any loc-tite someone may have used. After these two soak with Kroil or another thread release agent. Then attempt to turn screw. I like the drill press idea with maybe using vise grips to turn the screwdriver blade. Maybe chuck up a six-sided screwdriver bit and vise grips with drill press.