Casting spells until you run out of MP takes such a long time. Especially if all the enemies flee the battle before you've ran out of MP, or if you spend 5-10+ turns casting spells and then only your magic raises, not your MP. If you don't want to use the change trick, use the spell Sap. I think you can get it from Bafsk or the town where Josef lives (Salamand?). It drains the target's MP, and it's the way how I raised my MP very quickly. I think I had Frioniel and Maria with about 150-300 MP when I went to the Dreadnought. Frioniel was my white mage, Maria was my black mage.