When on the moon (whether you are with Fusoya or after he leaves your party), you receive the Bless augment. Equip it and ,when you encounter a Moon Maiden or Dark Sage, cast Toad on them with Rydia. Eliminate the rest of the enemies if needed. Then, cast Bless on your party to fully cure your party. Regain your party's MP by using Defend on all of your party members. The Toad only does 1 damage to your party member so just keep using Defend until you are all fully cured. Once your MP is full, kill the Toad.