For the rares that you guys may be missing, the only solution that I can see working is to endure the awful mode that is Hunter Arena. That was the only way I managed to complete my co-op treaure sets. If I had just stuck with Arena, I probably would still be missing treasure sets. Hunter is the way to go if you want to finish your sets. Hog all the chests, since that's where you'll get your treasures from 99% of the time in that mode.