TRIALS of MANA

All the achievements which description is "Become a....." are related to unlocking the Class 4 for each of the 6 playable characters, so you'll need a minimum of 2 playthroughs to unlock all 6 achievements.

After defeating the Boss at the end of Chapter 6 in Sanctuary the credits roll. Then you'll be asked to save your game. In the main menu now load up the save file (it has a little star next to it) and we'll begin the Epilogue.

You'll start before the final boss fight in Sanctuary and you'll have the objective "Head to the Royal Library in Valsena". There you'll be given the task of collecting the Sphere of magic (?)

For Charlotte you need to see the elder in Flowerburg Dior. He'll tell you that the sphere is in the Woods of Wandara. Follow the marker and you'll get to the fight. This time it's a regular 3 v 1 against Zombie boss that might summon regular zombies to join the fight.

After the fights, you get the Sphere. Head to Sanctuary to interact with the Mana stone to upgrade to Class 4 and unlock the achievement.