This is true to D&D, to multiclass into another class you have to meet certain minimum requirements that class would have at level 1. Generally speaking multiclassing isn't something you just decide to do on the fly but something you plan for from the moment you start character creation. Now in terms of the achievement of getting to level 12 with 1 level in every class...that sounds horrible. I don't even know if the game would be playable that way as you wouldn't be good at anything and you'd be quite miserable at everything. The only way I can think of getting that achievement is to progress as normal to level 12, make a manual save, respec your character into a mess that has 1 level in every class to see the achievement pop and then reload the save from before you screwed up your character. But to actually play the game like that...hell no.