I know that some of the challenges can be a bit tedious to complete, bro. But you'll really need to try it again. The unlocks for CoD games are system generated. Also, if this is the precision master, there are some other things you need to unlock as well. Here's a link I found about this calling card: Tips on Completing the Precision Master & Return to Sender Challenge in Black Ops 2 (HD) - YouTube
Look on the brightside though, atleast you'll just need 3 more.