I know that the final boss in Find Mii 1 is the ultimate Ghost. But what is the final boss in Find Mii 2?
If you are doing the regular quest in Find Mii 2, the final boss is the Dark Lord, which is essentially a tougher version of the Ultimate Ghost. He uses level down magic and is impervious to most kinds of Mii magic.
In the secret quest, it is the Dark Emperor, who is essentially a tougher version of the Dark Lord.