Is your MAGIC leveled? Magic is pretty much the key to defeating all the bosses in the game.
If you want to get real cheap, there's a trick by which you can "hold" enemies by casting magic really fast. While someone is having magic cast on them, they can't move. So while playing as the hero or the girl, cast a spell using the sprite, and as soon as the spirit or whatever they call them disappears, you can cast it again. Most of the spell animations are long enough that a few of them will get backed up and you'll have to stop for a few seconds to allow the damage total to pop up. Then repeat. It's cheesy, but it sounds like you could use a hand.
If you haven't leveled your magic, then I suggest going to the upperlands, in the place where there are the sections with the four seasons. Visit the temple there and have grandpa heal you, then just run the seasons in a big circle casting magic until your MP is 0. Your magic levels go up with every spell cast. In fact, to boost the girl's magic, you don't even need to leave the temple. Just cast support spells right there, heal, repeat.
I played that game so much back in the day that grinding was a meticulous science.
You haven't mastered the game until all of your weapons have a weapon level of 9 and 99 points on all characters.