Unless you get really lucky this can be a pretty hard drop to get. I've tried to farm this twice, once for me and once for a friend. Went 1/8 for me and 1/12 for him. If possible, I suggest just buying this on the Auction House.
The drop rate on this item appears to be 10-15% with TH1 which wouldn't be too bad, but the average spawn rate of this mob in the group of 3 is about 1 spawn every 15 minutes.
Therefore, it takes approximately 2 hours on average to farm up this item if no one else is after it at the time.
