Same here. I've asked this question few weeks ago. Since then, I have managed to get 5 in ONE predator, it doesn't work as a GLITCH, because for last few weeks i was playing mostly ground war just to complete them two. Many times i had at least 4-5 predators PER game, never acomplished the challenge due to glitch. Completed it only after getting 5 at ONCE.
Airstrike one is the hardest i think. Seems like i can see and predict accurately enemy's respawn but never get more than 2-3, maybe 4 kills of it. Too weak.