Get Sheela's boost spell up to level 2 and get all of your characters within casting range, and simply cast it. Wait until it wears off and cast it again - you can get her up to level 99 this way and she will have atleast 200 HP, 150 MP, 175 attack, 160 defense, and 225 agility. This makes her stronger than Volcanon. Try and select a random battle to try this trick in, like the one near Yeal. Choose a hard battle and you just might not make it.