This is the last of the 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 10,000xp to reach this level.
It is easy 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.