Instead of manually spamming Tidal Wave with Rita, try the following 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 "Item 
Thrower" skill. Turn off all other Artes for them so 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 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. 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. Finally, turn on the battle strategy. It 
should last roughly twenty minutes, and at the end you will receive 
approximately 800 Grade, with no work on your part.