As soon as you reach level 25 the achievement will unlock. The fastest way to gain levels are to do quests as they give a fair ammount of XP. There are certain perks cards that will boost your XP depending if you are in a team or if you play solo.
The CHARM perk card Inspirational unlocks at character level 2 and gives you
+5% xp at card rank 1, +10% xp at card rank 2 and +15% at card rank 3 while in a team.
Other than that, you gain XP from pretty much anything you do in the game. Cook food, craft weapons and armor, kill enemies, discover locations etc.
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