To unlock this achievement you simply need to gain enough experience to reach level 60. Defeat enemies and complete quests to gain experience and you should reach level 60 in about 40-50 hours of gameplay depending how distracted you get. Keep in mind that the higher the level the longer it will take to reach the next level. As you approach level 60 there will be a good deal of avenues to gain experience that will take you out of the normal story or region progression, but you can always go back an clean up older quests.