The Elemental Crystals are supporting bosses that appear in Super Mario RPG: Legend of the Seven Stars. They fight alongside Culex and compose of a the Fire Crystal, the Earth Crystal, the Water Crystal, and the Wind Crystal.
In battle, each of the Elemental Crystal have their own unique spell to use.
The Fire Crystal, as it name states, is capable of using strong fire based moves such as Flame Wall and Mega Drain. Though it is weak to Ice based moves, it's magical defense is rather high and is considered even more difficult to take out than the Earth Crystal.
The Earth Crystal has the highest amount of HP out of the four Elemental Crystals, though, it has a rather low magical defense. The Earth Crystal can use deadly earth-based moves such as Boulder and Sand Storm and it's weakness is thunder attacks.
The Wind Crystal is considered the weakest of all the Elemental Crystals but it has one of the highest defenses in the game. The Wind Crystal uses many thunder-based moves such as Static E! and Electroshock. It can also use the dreaded Petal Blast to turn one of Mario's party into Mushroom.