I once knew a guy who hunted boar. He told me that you had better kill it with the first shot cause by the second it will kill you.
Damned straight. Seriously heavy caliber, high stopping power, expanding bullets, and a good scope, because you want to kill that porker at considerably more than arms length away.
An RPG sounds about right. Or a AK-47 on full auto. A M-1 in a pinch, if you can hit what you are aiming at, and there is a tall tree nearby.
Wild pigs scare me.