I'm not sure if "level 11 class change" applies to all versions of FF1, but on all the remakes, it was the level you'd be if you only fought bosses and class changed at the earliest plot availability. It required a fair amount of resetting to ensure bosses with variable numbers, like the Piscodemons, were in their minimal configuration, and in my case, it required exploiting the living hell out of the ship minigame for spell casting items. I'm curious what parties people have achieved this with. There aren't many stipulations. If the game lets you do it, it's fine for this thread, even if it's cheap.
My only success so far has been Fighter, Thief, White Mage, Black Mage, and my version was the DS.
My current effort is Thief, Black Belt, Black Belt, Black Mage, and I'm already sick of the ship game. I didn't have fun with it on my prior effort, but as far as I know, there's no legit way to beat Lich in the remakes at level8 or 9 or whatever, without beaucoup de spell items. This effort is on the iPhone, which is the PSP version for all intents and purposes.