Just like @idosubaru says;
Turn the bolt a little bit, spray the bolt head with PB Blaster or other penetrant fluid.
Then torch the area of the head where the bolt is stuck with a map gas torch (handheld)
Turn the bolt back and forth just a couple degrees again.
Torch the area, spray with PB Blaster or dab on some candle wax while it's still hot. It will make the lubricant soak down to where it's needed.
Keep repeating the process until the bolt frees up