But don't slip.
One poster on another forum was outraged when he followed the pliers advice and slipped, damaging the thin steel tube badly enough that he had to buy a new tube and bar assembly.
I HAVE NOT TRIED THIS, and I'd be really careful about trying this due to the risks of damaging the part, BUT.......one person says he's used the gun's slide as a wrench to unscrew the nut.
Better advice is, see if your local gun shop has a fore end wrench, OR, make up a spanner from some 1/8" thick steel plate that will fit the notches.
Use some Vise-Grips to grip the spanner and unscrew the nut.