Instead of manually spamming Tidal Wave with Rita, try this method. It works almost as well and you do not have to do anything. Include Rita, Raven and Estelle in your party. The fourth member can be anyone with the skill "Item Thrower". Turn off all other Artes for them, so that they are only using "Tidal Wave", "Tempest", and "Holy Rain", respectively. For the battle strategy, set whoever has "Item Thrower" to defend, watch TP, and no over limit or fatal strikes. For the Rita and Estelle, set them to target different enemies, full attack and Artes, Artes only, keep formation, no items, over limit level 1, and no fatal strikes. Do the same for Raven, except do not use any over limit for him. As for special equipment, equip Blue Dice on Raven and the character with "Item Thrower". Equip Rita and Estelle with Risky Rings. Before the battle, have everyone set the "Minimum Damage" skill. Also make sure you have items to restore TP, such as Orange Gel. Also, make sure the battle difficulty is set to hard or higher for the Grade multiplier. Go just outside of Zaphias and link encounter either the plant or mushroom enemies so you have at least six enemies in battle to keep the over limit above level 1. Turn on the battle strategy, and there you go. It should last roughly twenty minutes and at the end you will receive about 800 Grade, with no work on your part.