Assuming this works the same as it does on PC and PS, good news! You won't have to suffer through a whole playthrough with a useless character to get this achievement.
Simply wait until you're level 12 and use Withers to obtain a Hireling. They always start as a level 1 with enough experience to catch up to your party, so if you wait until level 12 you can build one from the ground up choosing a different class every level.
Edit: In case it was unclear for anyone you have to press X to add class when on the level up screen.