This works in some areas. I haven't figured out a sure-fire way to grind Grade 5 Materials except Power Crystals.
-Rapid Fire Controller
-Rubber band (or Tape)
-A good set of Commando's (The stronger they are the better)
-Something else to do
1. First setup a Default Paradigm to COM/COM/COM
2. Equip Noel and Serah with Durable Collector's Catalogs. (A monster with Item Collector or some type of Item finding ability. You only need 1 COM monster so you fill the other 2 spaces with monsters that have those abilities.)
3. Head to Archylte Steppe -Year Unknown-
4. Make the Weather Sunny (both switches down)
5. Make your way to the very edge of the Clearwater Marsh Area (where you can see the cliffs)
Optional Steps: Turn On "Battlemania", and set "Encounter Master" to "More Enemies", Set "Clock Master" Ability to "High Speed." You don't need these but they speed up the process substantially.
6. Turn on your rapid fire to the X Button (A button for 360 users). Tape it down if necessary
7. Tie your rubber band or tape down both sticks. To either left or right. (The Right and Left stick needs to be in either left or right position.)
8. They should now be running around in circles encountering many monsters. It doesn't matter if you land the first strike or not.
9. Find something else to do and check back every now and then.
The point is you'll be getting in to battles. You should be strong enough to take out all the monsters there with just straight commandos. The most important monsters you'll take out....are the Moblins. The group of Moblins and the Major Moblins drop Power and Vitality Essences and the rare most sought after Power Crystals (Rare Drop). There's also Gancanaghs that drop Mana Esseneces along with rare occurences of Hedge Frogs, Metalicactuars, and Blue Chocobo's.
The only downfall of this method is that Noel and Serah may end up stuck at a cactuar statue, end up riding a chocobo, or somehow fighting Long Gui. You'll need to check back every now and then to make sure that hasn't happened and most likely fix it. Generally about 1 to 2 hours later I have 99 of Power and Vitality Essences and 99 Power Crystals. And about 500K gil.