Buzzy's revival: '82 Heritage Special

Long Socket ??

Using No chain guard at all =

1/4 or 3/8 drive QUALITY deep 6 point 12mm socket (not an impact socket) will get it loose, use a battery impact gun and you won't need to use a screw driver hold the adjuster from turning.
worse comes to worst grind the end of a socket a bit so it can reach fully onto the flats of the nut.