I did it, I got a rare item (some scythe) and the Bubble Belt(which doubles your HP), and the duke said that he dropped the area 11 level key into the ocean (im guessing its somewhere on the phon coast considering thats the only ocean I have seen so far).

I do have a save right before fighting em though(he actually wasnt that hard imo, I did 3 quickening chains, one being only 5 hits, to get rid of his fucking mantis compadres, so once it was 3 vs 1, I just kept healing and esuna....he went down eventually).