I found a good method to break it. In the gutter where the giant "Gas emitting Bug" that heals poison is. When you enter the cave and are facing the bug, walk to your right, to the rear of the bug. 1h R2 or 2h R1 against the wall. It can take 10 points of durability per swing. It can also randomly do only 1 point of durability. Its kinda weird sometimes.