It takes practice to get those camo challenges for the launchers (or any weapon, for that matter). NEVER expect tol unlock all of the camo for a weapon in 1 day. Try experimenting in different modes. Like mentioned earlier, Domination and Hardcore Kill Confirmed are great choices if you're trying to get those double kills. Don't expect to get them every time, however.
It's considerably easier for the RPG challenges (to me at least) mainly because you start with 2 rockets as opposed to the SMAW's 1.
Misete agemasho... Karnoffel no shinsui wo!
(Let me show you... the true essence of Karnoffel!)