The Challenge that requires sniping the driver of a moving vehicle and saving a teammate from afar is simply too difficult. It’s hard enough to fight for that sniper rifle from others, and you might spend the only few bullets it has on enemies before you even see the first vehicle coming, at which point it’s still very hard, you really need to be lucky to land that shot. Same goes for the second one, you really need to be lucky before anything, and what does “saving” mean anyway. It just doesn’t seem very fair to depend so much on luck to complete challenges.
I know some of the better players out there won’t relate to this, but me and my friends have been trying for almost 20 BTBs and still not completing it.