Spray penetrating oil on the rusted nut and let it soak
Clean off loose rust with a wire brush
Use the correct-size wrench or socket for a tight fit
Apply steady force to loosen the nut
Tap the nut lightly with a hammer to break the rust bond
Use heat on the nut if safe to do so
Try tightening slightly first, then loosening
Use a breaker bar for extra leverage
Use an impact wrench if available
Cut a slot in the nut with a rotary tool
Split the nut with a nut splitter
Replace the nut and bolt after removal
Wear eye protection and gloves