I think the pistol challenges should have been halved. 50 headshots, 15 revenge kills, 75 kills no perks or attachments, 10 double kills and 5 bloodthirsties.
I attempted to get diamond pistols but trying to get headshots with the executioner is a real pain. There have been time where I've snuck up behind someone looking in a window, ADSed at the back of their head and for some reason it doesn't count.
There are also a couple of other poorly thought out challenges such as getting a bloodthirsty with a crossbow (really Treyarch 
), getting a triple kill with an RPG, destroying dragonfires (a streak that nobody uses unless they get it in a skill package).