You can do two things:
1. Do the speed fire team leveling. Easy, not very fun, but it gets the job done.
2. You can try and do a 4 man fireteam to begin, and if possible, get at least 1 to leave, better if all leave and you're the only one there so you get MAJOR EXP in Fireteam extreme. I do it often, it is literally HARD AS SH*T, but so worth it, because if you make it to Round 10, I guarantee, for now, you can get more than 1,000 GP and/or EXP.