This is the 15th of 16 levels and you will reach it through the natural progression of the game as you collect experience for killing creatures, completing quests and challenges, and finding xp-rewarding items. You need 8500xp to reach this level.
It is possible to complete the main quest without reaching this level. However, you can keep playing after the main quest is over so there is no way to miss this. If you complete all of the side quests and challenges you should get this without any problem.