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 venture to places with plenty of enemies youve 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). Some good places would be:
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 bases appearance)
Hint - Dont 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. However, there are two accessories you must steal from two different bosses or you will not complete your item collection.
Be sure to steal from the Phantom Dragon south of the Glass Spires on disc 3 for one accessory.
Be sure to Loot or Steal from Ultimate Szabo in the Primitive Cube (Cauldron Room) on disc 3 for the second accessory.
Hint - Rare Monsters R Us
Aside from the 20 to 30 monsters you can miss in Nenes Fortress (end of disc 2), there are three elusive monsters you will have to search for once your party gets a mechat on disc 3.
These monsters are super tough and can do more than 999 damage to everyone in your party. Increase your shadow class ranks or expect to die very quickly.
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 Platnum 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.