Basic blender went bad (motor ran but spindle wasn’t rotating). I wanted to disassemble to see if it could be repaired. Three of the four screws were Phillips head. I had to cut the casing open in order to discover why I couldn’t unscrew the fourth. It was a slotted spanner.
Grind a flat
How are you supposed to grind a flat on a screw recessed 2" in a hole?
Take a flat screwdriver and moubt it on a vice. Take your angle grinder, mount a disk for metal grinding. Grind away the middle of the flat screwdriver head. Done it before for the exact same purpose.
1" Grinding wheel. Duh.
I didn’t even know that that screw type had its own name until I saw it in another comment, but the first thing I thought was to just do what you said.