Best to get the flywheel holder if you can.
If you put it in gear you need to hold the rear brake on to stop the wheel turning. Sit on the bike with your foot on the brake. Get the socket and bar on the nut and apply pressure to take up the slack, then give a sharp pull to try and snap the bolt free. It will usually work this way. Sometimes the clutch will just slip, as Fontyyy says. Try backing off the clutch cable adjuster if it slips. Anti-clockwise to undo the flywheel bolt!