The final form of the Chykka battle, the adult imago has been corrupted by the darkness. Aim your arm cannon as best as you can and lay into its lower abdomen with Light Beam shots. Use the nearby Chykklings that fly at you to replenish your ammo if needed. If you run out, just use Charge Beam.
Fortunately, Dark and Normal Chykka forms share the same health bar. Dark Chykka will appear when Adult Chykka has lost 1/4 of its health. It will revert back to normal Chykka at 1/2, and then once more back to Dark Chykka at 3/4 depletion.