TreeClimber's advise is correct- for a shot with an arrow, it's best for a quartering shot behind the shield.
I've included a pic from the TexasBoars website that shows the ideal killing zone for gun hunters so you'll see where they are aiming for.
http://www.texasboars.com/index.html
a great website for hog hunting

CaptJack
Be a better Hunter - Get a better Shot