Here is a small list:
1. Chalenges, not only are they fast and easy you can do them over and over and over
2. Old missions, you already know you can do them why not do them again
3.Just handing out stuff like buffs ammo etc. (slow but you dont die)
4.Free Play, on or off line
5.New levels, it keeps you interested in the game and how it works also willl get you ready for online and if you have online do campaign on co op it will save your life!!!!

Please reply i want lvl 20 for anger outfit lol
HOPE THIS HELPED YOU REACH YOUR GOAL LEVEL
(yes the caps were intentional)

Buffing as a medic. You can get 1000XP 5 minutes into a match with ease.

kk thx

Check this out:

http://www.gamesas.com/index.php?/topic/1210257-lots-of-points-each-match-potential-spoiler-inside/

Yeah, play as a medic in any game and if you're attentive, you can EASILY finish most rounds with nearly 10k in exp... Engineers too if you focus on buffing your allies. It's faster as medic though.

Play a defensive map as a medic and escort a primary objective class, while getting a ton of kills. Buff = +xp, defend = +xp, escort = +xp: Then if you accomplish sticking with your combat buddy and defending the primary objective all the way to the end of the time clock you get a nice point bonus.

Yeah. What he said.