AE Dial with stuck fingerwheel screw

[Volvoguy]

I have an AE50 that I am fixing up. I want to take the fingerwheel off, but the big flat blade screw is quite stuck. Looks like somebody before me tried to take it off as well, as it looks like a screwdriver has slipped a few times and wrecked the edges of the slot a bit. I tried and might have slightly added to the damage. There is still some there though, enough for a screwdriver to hold on to. My question is does anyone have any suggestions before it gets more damage from more trying?