i cant get this undone to get 5th off my xsi box

got the gears locked (reverse and 5th engaged) and tried hammer and punch but i think i need a c spanner unless anyone else knows a way?
guide has a 27mm nut there....

got the gears locked (reverse and 5th engaged) and tried hammer and punch but i think i need a c spanner unless anyone else knows a way?
guide has a 27mm nut there....