Complete the game to unlock the "Bonus Mode" option.

For air: when he isn't attacking shoot at his three circles.
For earth: keep avoiding his attacks when he bends down and his faces turn red hit his faces. When he falls apart hit nine of his minions.

For fire: between attacks stand on the aiming circle and hit the target behind his head. After he falls down keep hitting his 2nd head. All of them will eventually explode.