Odin's Gungnir in Final Fantasy 3 DS.
If I remember correctly, he can be fought around mid-game. Around the time when you're level ~35. Odin has a 2% chance of dropping Gungnir, one of the most powerful weapons in the game. Problem is, there are no save points in dungeons. If he fails to drop it, you have to reset and go through the dungeon again to get another shot at fighting him. It can be easy to get worn down before making it back to him, and Odin is a decent threat
Alternately, if you somehow manage to have a thief with a class level of at least 71 at that point, you can attempt to steal it from him with varying success.