I don't think it is possible, if you don't have the code you can't play past level 5. Now if you have the code you are good, even then it really just depends on how good you are. I would recommend playing Seize Ground and capturing points to really get a good amount of XP. However you could always try to find some people to boost with if that doesn't bother you.
Vision? What do you know about my vision? My vision would turn your world upside down, tear asunder your illusions, and send the sanctuary of your own ignorance crashing down around you. Now ask yourself, Are you ready to see that vision? - Huey Freeman