I was helping a friend with this yesterday and got one off the end boss at my 4th run through. He was at about 25 runs with only one for him so far (also from the end boss). It does drop there, but the drop rate may be lower than other similar boss drop leads from dungeons, or at least that is our experience with a sample size of about 30 runs. Hard to say without too much more data. All our runs were on normal if that matters.