Kind of hard to do challenges when every lobby is sweaty as hellHaven't you been done like 10 times already?
Anyway in terms of the BP the blog on Activision site says it's a combination of Time Played, exp gained, and completing challenges. That why there is such disparity between how people are progressing. People who are constantly completing challenges are leveling up fairly fast while others who are just playing matches and not going for challenges are going much slower.
I've been needing killstreak kills for days now. I can't even get a damn UAV nevermind airstrikes and predator missiles.