i have a friend who is in a way credit boosting i guess... but how he's it doing doesn't seem like it goes against anything. i mean what he does is for some of the campain he waits for a checkpoint then kills jackels or grunts, elites etc and reloads it. but he does it for like 10- 20 minutes then stops so he can get the the completed challenge.

can you be banned for that? boosting in the campain for the challenge? (ex: get 70 headshots on night fall) then stopping?

no u cant be banned at least i dont think so and ya it might boost credit but it does get boring he probably will stop after awhile