You need approximately 7 million xp to reach level 30. Finishing the Shatter Bay Career will put you roughly between level 20-22 and this is only around halfway in terms of xp. You will have to play extra events online to earn the remaining xp.
Go to World Cities in Multiplayer and search for the creator "igor5618" click on the "XP Boost" city and choose Lot XP Frag. Playing this event until you have destroyed all of the tankers, and then letting the time run out will net you roughly 220-250k xp per run. It will take around 10 minutes to destroy all the tankers then you will have to idle for 3-4 minutes on top to let the game end. This means you will gain around 1 million xp per hour, making level 30 much more reasonable.