There was this one guy with a blue arena shield and I kept chop chopin on it with a long axe (think I had around 7ps) and it didn't break. Must have hit it at least 7 or more times . Eventually I just left and moved on to another foe because it was getting comical and ridiculous. Don't know if it was a bug or not (like that invincible bug that happens once in a while). Maybe his shield eventually broke..or not. Must have been crafted out of enchanted elven oak or something, lol.