1) Clear Hero 30 - Going through Last Battle and The Hero of All Time give you the same ending sequence.
2) Side with Mo in Quest 22, The Have-Nots (Beach branch of Quest 21, Floodgate Panic)
3) Defeat the Undead Queen (Quest 18, land branch from Quest 12 Road to Land and Sea) without a grail - no special ending is shown, but it boots you back to the menu.
4) Don't find enough treasure in Quest 17, Ancient Riches (Devil branch of Quest 14, Nun and the Devil)
5) Don't defeat Cully in the cave in Quest 9, Change the Current
6) Find and take the Pirate Treasure in Quest 16, Treasure Island (Ocean branch of Quest 12, Road to Land or Sea)