Bit of a pain to farm if you're unlucky, however i was somewhat lucky... at first. I got my first one in around 5 kills, however. I'm starting NG+ soon so I wanted to trade it in for the shield and get another one for future farming and it seems RNG was not with me. Took another 2 hours to get a second one to drop. So goodluck farming, all! Hopefully you get lucky and it drops sooner rather than later.