If the nut is rounded off, take a coarse file like what you might find on a leatherman and file down two opposing sides flat. Spray the nut with some WD-40, PB Blaster, etc. Clamp a set of vice grips on there as tight as you can physically get them. Tap the vice grips with a hammer, but don't smack them. Tap them for 30 seconds to a minute, then hold as much pressure on the vice grips as you can hold without allowing them to slip. That doesn't mean as much force as you're capable of applying, but as much force as the vice grips will take without slipping. Hold that pressure for 30 seconds to a minute, then apply some fresh WD-40/PB Blaster, tap with a hammer, then hold pressure, rinse and repeat.

You might even try heating the nut with a lighter to get it to expand a little bit.

I've used this method to remove old worn out brake bleeder screws that were rounded off and after about 5 minutes of tapping and holding pressure, they've always come loose.