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.