Here's how you can get some easy Grade. First, put the battle difficulty to Hard or Unknown (the latter is only available after you beat the game). Then, after beating Nebilim you should acquire an accessory called Blue Dice. Equip them on someone (coupled with the Normal Dice for even more Grade), and go to the Fallen St. Binah area. You should get around 7.4-8.4 Grade per battle.