This trophy, as with most of the leveling trophies, should come naturally as you play through the game on Normal mode. Killing enemies, completing quests, and crushing challenges are the main ways you gain XP in the game. A helpful note is to make sure you finish all the area Challenges in each area of the map. Area Challenges can be found by navigating to the map screen and then pressing the right button of the D-Pad. Challenges are found by wandering around the map and looking for icons related to the challenges that are available in that area.