Well considering the White Magic spell for Multi heal originally needed Sage/White Mage LIKE AT LV 34 to even USE it Song of Peace at lv 6 will pretty much LAST for the entire game until Multi heal for your Priest/Sage becomes usable.
But I'm not even sure you CAN skip that towers boss fight IE get the Magic Key in the Pyramid area then use it to open up Portuga and go their then go towards Dharma Temple itself then farm the HELL out of the Metal Slimes from lv 15 to lv 40 THEN Crush Kandar in the Tower of Najimi at lv 40 with every white/black mage spell learnt on the 3 Non hero's. But this remake may FORCE you to kill that boss before you can possibly use the Magic key itself.