Use PB Blaster, Liquid Wrench, etc. like mentioned above. If no impact...give bolt head a good hard tap and then put the socket/rachet and a long pipe/or box end wrench on the handle of the rachet. Either it breaks the bolt or loosens the nut. That's the way it works. Bolt/nuts can be replaced.
Sometimes: tightening the nut 1st (slightly) and then trying to unloosen the nut will help in some cases.