AR-10 in .308 is pretty popular.
.308 (a.k.a 7.62x51) I would consider as a minimum. 5.56 would require several rounds, in the case of Hogzilla, piss him off. I have heard of some people considering a rifled slug from a shotgun, but the range is not ideal.
For safety, I would prefer some good range and hitting power, since I prefer to have some distance if Hogzilla and friends decide to charge.