Seven Star Ruin

After beating the final boss of the game, use the oracle skill, and go through the south-west door.
You\'ll end up in the Tataroy arena. Now go one step up, to arrive in the Seven Star Ruin. This secret dungeon has 30 levels and no save points.
It contains super hard enemies, and a even harder boss.
At the bottom is some of the best items in the game.

Invincible characters

This glitch allows you to make your entire party (except for Ratix) invincible. It works by reducing them to 0 HP outside of battle but not actually killing them. As a result, the game will treat them as still being alive, but not apply the effects of any damage to them.

Requirements:
One character with either "Rhythmic sense" or "Pitch Sense", but not both, and any music level (higher is faster).
One character with neither of those talents, and level 1 in music
Either Guitar song or either Trombone song, and the appropriate instrument.

Step 1: Decrease everyone's HP to 1. Best way to do this is to use a Guitar song (preferably 2) or a Trombone song (preferably 2) with the first character. You should hear a chirping in the background, and your HP will decrease. Continue doing this until everyone's HP is at 1 (Ratix does not have to be at 1 HP).

Step 2: Heal Ratix (or at least one character) so that he does not go to 0 HP in the next step.

Step 3: Have the second character use either Guitar or Trombone song. Everyone's 1 HP will go to zero, but the game will still have their walking sprites. You can neither heal nor revive them now (although you can recover status ailments).

(This is the glitch)
Step 4: Enter a fight. The 0-HP characters will still be active and attack...and receive attacks, but the enemy attacks won't ever connect. Milly and Joshua will neither heal nor revive the 0-HP characters. Note that if all 4 characters in the battle reach 0 HP, it is still a game over.

After the fight, the 0-HP characters will still receive EXP, and they will recover HP and MP due to Stanima (so you have to reset the glitch). But the important part is that they cannot be killed or even touched by any means.