Here is what you do.

1. Spray the bolt with PB Blaster (let it sit for 5-10 mins)
2. Hit the bolt with a hammer a few times
3. Spray the bolt with PB Blaster (let it sit for 5-10 mins)
3. Hit the bolt with a hammer a few times again
4. Try and break it loose with a breaker bar and an impact socket (u might break a regular socket)
5. Use a propane torch or a torch of some sort.
6. Hold the torch on the bolt for about 5 minutes or so.
7. Break it loose.

That is a fool proof method to getting the bolt off. If you want to be extra safe (you really should) so it doesn't happen again, apply some "never seize" to the bolt when you put it back in.