Red Faction Guerrilla

Earning XP is rather simple, but you don't have many events that give you XP at this point. Playing siege (once unlocked, of course) will net you quite a few points as long as you STICK TO THE OBJECTIVES.

Just remember to always do the objective required to win. Getting a kill is not as important as capturing your flag. If you are playing CTF, hunting down the enemy flag carrier will give you more points than a standard kill.