picutre of the lockers might help

otherwise,, put a socket onto the end of the nut and use as a guide to drill a pilot hole
then go through with bigger and bigger drills until you drill the nut off

start off with around a 3mm drill to make the original pilot hole
then go up to a 6mm, then a 12mm
the thread in total is around a 14mm drill to clear, but with 12mm you should be ale to sort it

you'll also need a proper drill, none of this cordless stuff tends to be up to the job