I'm now very pleased with myself :)
Been trying for a few hours, using the 'bait' method, and after a few frustrating misses, as well ruining some with my own itchy trigger finger, I finally got it, and it was FUNNY.
What I did was play Free For All, got 29 kills, then kept throwing care package markers (saved up about 5) in the same place every few seconds hoping someone would try and steal them, and it worked! I'm just glad that I waited until Prestige 10 to do it, because I never want to try that again!
But it's a good method for getting the GWK challenges in general, cause you know that the next kill you get will be the winner. You could probably use the bait method for the airstrike and equipment challenges as well.