There are several methods that can be used, and they can be used in sequence if necessary. I list them in order of complexity (simple to complex):
Soak the bolt with penetrating oil for 48 hours.
Apply a short tightening (yes, tightening) impulse torque such as provided by an impact driver
Apply an impulse torque in the loosening direction, such as provided by an impact driver.
Apply a slowly increasing torque in the loosening direction, over a period of 48 hours.
Heat the joint with a torch.
If possible, use electrolysis.