This can be obtained at the very begining of the game.
Shortly after a cinematic, you'll be following a character to the "Adventures Guild". There the tutorial will ask you to continue as a Bard or create a customized character where you'd be able to select the character's class. Select the option to create a customized character and create which ever you don't plan to use for your game.
When you finishn the character creation, you'll unlock the first achievement and you'll be in a dungeon. Reoad the last savegame (the game has saved automatically) and you'll appear directly on the character creation screen again. Repeat three more times, leaving for last the class you want to use for your game. This way you can unlock: