i found them all pretty easy, sure a bit time consuming but nothing difficult. Usually i would find one challenge each that is very hard to do
i.e
Ghost
Destroy 1 sentry turret (not many people were using it as a killstreak, and they also camp with it to protect it)
Flack Jacket
Throw back 5 enemy grenades (again its all up to luck, many people perfer semtex or cooking them, making them hard to throw back when prompted)
and suprising hardline
share 10 care packages with team mates (it took me a while since, for once people wouldn't want to take them xD, easier to gain when you have sam turret as well as care package as killstreaks)
and with the CTF medals, camp around the flag, and grab it as soon as you can, and run back along the sides with cover. Also follow team mates who capture it, as when they die, they'll drop the flag letting you grab it and get it home. It does take some time to figure out tactics, but its easy by the end. And another pointer, if you find camping at the flag impossible, find another lobby after the game is done