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.
I’d use my dremel to finish the slot that was only partially cut.
Or use the dremel to cut a slot in the end of a flat screwdriver.