Hint: Rare Monsters
Considering there's no real 'cheat' cheats for this Final Fantasy clone, we can only provide hints, but this one's a good one. There are three rare monsters you will have to search for once your party gets a mechat on disc 3. These special baddies are very strong, able to lay waste to your crew - they can inflict 999 damage to everyone in your party. So before you search them out, make sure your shadow class ranks are high.

Gold Mecha Robo - The Ancient Factory 1F. Past the first conveyor belt.
King Poo Snake - In the Ancient Ruins Cave. Defeat the Jumbo Poo then the Corrosive Poo for this enemy to appear.
Gold Poo Snake - Defeat the King Poo Snake and the Platinum Poo. The Platinum Poo is on one of the three south and southwestern island archipalageo and the Gold Poo Snake will rarely appear (on that same set of islands), giving you that chance to destroy it.

Hint: Easy Kills
The Barrier Magic has a field skill "Field Barrier 3" which only costs 1 MP (instead of 10) at rank 50. Once you earn and assign this skill, you can explore places with lots of enemies you've defeated before and simply walk into them with the Black Belt and Eyepatch accessories (the skills Attract Aura and Regenerate MP at this point are self-explanatory). Examples include:

Underground River - the Kelolon Defender Room, all the way to Baroy Town (80 enemies)
Primitive Cube - the Blazing Moths in the garden (infinite but slower number of appearances)
West Ice Field - the Roball racetrack (dependent on the moving base's appearance)

Hint: Don't Miss Out
If you are looking for the "All Item Achievement", you will be looking for all the 1800 "nothings" and trading them to the Nothing Man in Jibral (or the Nothing Robo Mecha in Split Worlds) for items. Exploring every other chest in the game will pretty much help augment your item collection but there are two accessories you must steal from two different bosses or you will not complete your item collection.

- Make sure you Steal from the Phantom Dragon south of the Glass Spires on disc 3 for one accessory.

- Ensure you Loot or Steal from Ultimate Szabo in the Primitive Cube (Cauldron Room) on disc 3 for the second accessory.