This is kinda spoilery for Zelda II if you don't feel like reading it, but mandatory for the Great Palace boss and I don't want you to get so frustrated you break things.

Great Palace BossYou need to save your magic instead of casting Life and whatnot to help get through the dungeon. In order to hurt the boss you have to cast Thunder at the start of the fight and it takes a lot of magic. You may be able to cast a Life spell during the fight afterwards. I don't remember if you have enough magic power left, but I want to say no. More importantly, with the remaining magic, you can cast reflect towards the end of the fight, instead of dodging all the damage while finishing it off. I remember there being a fairy in that maze of a dungeon to help you stay alive before a long elevator trip down, and that was always helpful, but I can't give you any clearer direction than that.