I agree on the quartering away shot.
And just to help those out on the sheild issue:
From what I' ve heard, the sheild is scar tissue formed from the larger boars fighting with each other. I' ve seen it on pigs from up in my treestand as they went by (as recently as a month ago). The bigger boars usually have it.

The best example I can remember was a riffle shot pig on my lease that was so big It hung from the top of are skinning rack and it' s head was laying on the ground. As they skinned it from butt to head, the skin wouldn' t even fold down along side the body. It just hung in the air at a 90 degree angle when they got down to the scar tissue/ sheild area. It must have been 1.5" -2" thick. I remember saying to myself " there' s no way I would have even tried to shoot that thing with my bow" . I also remember his tusk/teeth were so sharp you didn' t want to apply preasure when touching them. They were sharper on the tip than the sharpest pencil you can think of.