Stand in the opposite corner of the witches and use the Mirror Shield to hit the opposite witch. You can manipulate the twins somewhat by shooting the Hookshot at them to get them to spin in place, making it a little easier to bounce back their attacks. There is a chance to get what is called a double or even a triple when one of her attacks twice (or 3 times) in a row without moving. One of her can also do a quickshot at the beginning of the fight, meaning she will atack before she starts moving. Phase 1 ends once 4 hits are landed, however the fight will continue if you get a double after the 4th hit, if you see the witch staying in place, just hookshot her to end the 1st phase.
The three 1st attacks will always be the same element (fire or ice) so you simply have to absorb the attack. After she throws her spell, you can use the hookshot then quickly shield to absorb the spell and to make her move faster. When the Mirror Shield returns the magic attack, jumpslash then mash or time (faster) your crouch stabs until Twinrova is dead.
There is a way to create an invisible Twinrova if you kill her with her spell instead of your sword, then you have to wait a long time before an invisible Twinrova appears. This is the only known way to duplicate heart containers.
If you are skipping Requiem, you should obtain a warp song first since the death cutscene skip will put you back at the main entrance.
In order to conveniently skip the Spirit medallion cutscene, you must die as the cutscene starts, so you must have 1/2 heart or less after defeating Twinrova. If you have enough hearts, you can use the Nabooru miniboss to quickly reduce your health. Otherwise, just use explosives.
Once Twinrova is defeated, catch your bugs/fish in a bottle and move to the blue warp. Once in the right position, ocarina items and wait until your camera returns to you. For pauseless setups, once the camera is correct stop the ocarina and hold a direction. Link shall climb up the platform after a jump and if necessary adjust your direction and pull the bomb at this point. After some point in time, let go of the control stick and Link should hit the ground and die. Some examples are shown below.