The best strat I got for her was to brute force it as hard as you can with the axe and fire spells. Just make sure the main hero has full health when going in, which means you'll need to get from the water fall to the boss using the magician character. I stood in her face and spammed Z and X until the problem was solved and left, just beware that the giant tentacle mouths do not despawn when she dies and you'll still need to avoid that to get to the ladder out.
Again, do not let the boss character kill the magician or the game becomes bugged where you can't damage the boss or leave the room.