Crossbow: The only 2 camos that were frustrating were the skulls (Get a kill with every bullet in a clip) and Ronin (1 Bloodthirsty) camos since I reallly don't have good aim with the crossbow and you start with 6 bullets. I got both in a S&S game after 2 weeks. My opinion the hardest gold camo in the game.
Assault Shield: Played Hardcore TDM for this one. It did bring down my K/D a bit since it can be annoying just to get near someone. I don't think there were some really annoying camos I had to go through besides the 25 kills w/o weapons or perks (Ronin & Skulls), but it did take alot of patience and I eventually got the gold camo.
All I need is the ballistic knife gold then I got diamond camos for specials.(Yes I have the combat knife gold as well.)
Knife people in sticks and stones for ballistic knife.
Also when you get to one of the specialized camo challenges, it calls for getting projectile kills that have been recovered. At every life shoot them out and pick them back up. Then any kill counts toward it.