A super genious on here suggested making pig pits instead of fences. SO that is what I did, and it works well. Stuff sometimes attacks your fencing. The boars may themselves if they get frightened by a mob. So if you don't use pits, you have to check/repair your fencing maybe once a week, or every other week - whatever case may be.

I capurted a 2 star and checked the fence twice to repair before he was tame. I had a raid and greydwarves seemed more interested in getting at my 2 star boar that was being tame then me. That was fun as I had to kill the mod without knocking a fence down as they were right next to it

My training pen is still fenced, but them I push them into a channel that drops into the pig pit.