Phase 1: Elemental Absorption
Twinrova consists of two witches, Kotake (ice) and Koume (fire). They will fly around and shoot elemental attacks. Use your Mirror Shield to absorb three consecutive attacks of the same element (e.g., three fire blasts). Once absorbed, reflect the charged energy back at the witch of the opposite element (e.g., reflect fire at Kotake, ice at Koume). This will damage them.
Phase 2: Combined Form
After taking enough damage, they will combine into Twinrova. She will shoot a large elemental beam. Absorb this beam with your Mirror Shield. Once your shield is fully charged (it will glow brightly), reflect the beam back at Twinrova. This will stun her. While stunned, jump across the platforms and attack her with your Master Sword. Repeat until defeated.