Having to control 4 characters, each with their own powers, in a tower defenseque cenario is a really neat concept.
However, I never really felt the need to swap them. I believe the game would benefit greatly from mechanics that force the player to swap around, such has having special enemies (a fire ant that can only be harmed by the air elemental, for example) or multiple crystals away from each other (so you can't just follow it around with a single character). The "mix attack" mechanic is a good ideia, but it feels more efficient just to spam a normal attack.
As a side note, the art is cute. I liked it.