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To remove a rounded nut, use a pair of pliers that have curved jaws and are serrated, such as locking jaw pliers, to turn the nut counterclockwise. Alternatively, you can use a file to flatten 2 sides of the nut so you can grasp it with a wrench to remove it.
Considering this, how do you remove a rounded nut?
Stay away from needle-nose and flat-jaw locking pliers though, as these won't be able to get the grip you need. Instead, use the round section of the jaws. Often a cause of rounded bolts, locking pliers are the go-to tool to remove them as well. Make sure you crank it down tight.
In this regard, how do you remove a seized nut?
Try a 6-point wrench or socket on your seized nut/bolt. Start by rocking the bolt by tightening then loosing, this may be all you need to break through the rust. Try and avoid 12-point wrenches and sockets as they likely to slip and strip the bolt head.
Heat everything, cool the stuff you want to shrink, remove stuck nut/bolt. A fastener is not freed just by heating it. As in, you can't make it glow, then walk away for an hour, and expect it to turn when you come back. The parts will only turn when they're hot.