or you could just join a game that is currently playing that map (or the one before it) that you need to get the objective. (ao you are basically joining someone else's campaign, and it helps you)
one time i joined a game... and we were already playing and they told to me get the fcku out and im like what??then they started shooting me.. so i then then started to empty 150 clip's into they head wit han f90.....i mean i had the most unlocked gun.s and they want to kick me out........oh yeah