Okay,
1. Complete Challenges. You would get, I believe 41,600 xp if you completed all of the weapon challenges, (Marksman and Expert), plus a lot more for the Boot Camp, Operations, Killer, Humiliation and Elite challenges. I'm not too sure on the total xp you can gain from all.
2. Play HQ. Headquarters is a great game mode to get kills and points. Even though kills are only 5xp each, you can get over 40 on average. You also 15 xp for capturing OR destroying an enemy HQ, and you get 5 xp every few seconds for having the HQ captured.