When you get all your characters Art moves, including the Hyper and Miracle arts, go to the original tree the character got his/her Ra-Seru to get the Meta/Vahn's spell. Terra/Noa's spell, and Ozma/Gala's spell. Also, when you get Cara's Ra-Seru egg (give the music she gave you to the pianoist in the Jazz Club, then go back to Buma and talk to Cara) give it to Zalan in Jeremi. He will give you an accessory that let's you use the Horn spell, which refills all HP, even if you're dead. However, it costs a whopping 200 mp to cast.