When a nut is rounded, there’s only a few ways to safely loosen it without damaging the thread.
Here’s a way that involves filing down 2 sides of the nut, so you have 2 flat edges, then with a close fit spanner and a wedged in screw driver, you can then have the leverage needed to open it.
Hex High Tensile Bolt SAE J429 Grade8, Material: 40Cr, From Ningbo S&D Metalwork Co.,Ltd, www.sendingmetalwork.com