use some penetrating oil on the threads. PB Blaster or Liquid Wrench. Let it soak in at least 24 hours. It there is a lot of corrosion on the bolt do multiple soakings.

It there is enough exposed threads on the bolt thread on two extra nuts above the frozen one and tighten them against each other. Then use two wrenches - one on the double nut and one on the frozen nut to try to work it. If you grip the bolt threads with a vice grip you will damage the threads ans will never be able to get the nut off.