One way which you will be able to level up quickly is by taking part in the lightest games in Quickplay where you are likely to get teamed up with players that have less experience than yourself. Quickplay lets you jump into teams that have just one slot free, meaning you can find yourself in any mission or stronghold in the game. If you help the team to complete the mission you will recieve XP bonuses. Even if you enter the game at the last moment it is worthwhile, especially if it is a mission you have already played as you will still be eligible to recieve a few XP more. If you do this playing with friends you will find you can reach level 30 easily in a couple of hours.