Reach Maximum Level and Become the Ultimate Badass
The maximum level in this game is 30. You gain XP for practically everything in the game, such as killing enemies and animals, liberating Garrisons, completing sidequests and finding collectables. By the time you have completed your clean up of the island, you will easily have earned enough XP to reach the maximum level. If by some chance you haven't, you can still earn XP post-game by killing the few enemies and animals dotted around the island.