I'm a little confused. A battery post is connected to the battery and does not turn. So, I assume that you are either talking about a battery post cable terminal or a battery post connector that has a machine vertical post on it the you connect the battery cable to and tighten down with a wing nut or regular hex nut.
So if the battery post's cable terminal is off of the post and the bolt is one that goes through the prongs of the terminal that will tighten around the post with the bolt. If this is the case, I would put a wrench on the head of the bolt and vice grips on the nut and pull upward (or outward) while turning counter clock wise. If that doesn't work, I too would try to cut the bolt in half between the terminal's prongs.
If it is the battery post's terminal with the vertical machined post and it's the wing (hex) nut that is stripped, I would still try to use the vice grip method....or just cut the post off and buy another.
Hope this helps
Ron