Like a lot of games, you will gain experience as you defeat enemies and complete quests. Once you have accrued enough experience, you will level up and get stronger. The max level in the game is level 50, while you only need to get to level 30 on CJ for this trophy/achievement. This will likely unlock naturally as you complete the main story, so you shouldn’t worry about this too much. There are accessories you can equip to boost your experience gain, as well as the temporary buff from the Inn.