On this floor is one of each kind of Phantom. Since you need the warp phantom to open the door, you have to control it. Make sure you are at the left side of the room, shoot the phantom eye, and the noise should attract it's attention. When it's near, shoot the wall across from you ( make sure you have a bunch of arrows ), and when it goes to check out the wall, it's back should be turned to you. Charge your arrow, and shoot it in the back, then take control. (; it's easier than distracting the normal phantom with Zelda and sneaking up from behind.
It's actually fairly simple and yet complicated. All you really have to do is pay attention to the "Tempo" but the funny thing about it is that for me it messes up fairly easily. (gets mixed up with an off beat rhythm) none the less all you really have to do is to try to play your notes accordingly. If you have a simple song like the first 2 in grass and ice (supposedly called "wind" but I believe it's actually ice because of all that ice theme!) all you really have to do is simple slides right? Easy enough and hard to fail at (if your trying anyways).
But if you have a "complicated" song that requires you to skip a pipe to get to the next like in the Ocean realm (or fire ... Currently on that though so no details on that from me! ... Yet) all you have to really do is slide quickly past the pipes not required and stop at the one you need. (that means do not stop blowing just because 1 pipe you shouldn't play is in the way) and you should be good, the way it sounds to you will be butchered and messed up but there is a high chance it will work out for the game. (if done properly)
A few side notes of "do not do" because for some reason it affects your "result." remember not to "Put down" flute (which is done by releasing your stylus from the screen)during your turn. Do not do a short breath for each pipe because that is not only time consuming but tends to affect your end result (dont worry about mic stutters though, but do try to avoid those), if there is 1 pipe that shouldn't be played just try to slide past it quickly, that means do not stop blowing and start again for the notes you need. Do all of that and follow the simple rules and you should be fine
Quick and simple version (i reccomend you read the long if you do not understand): Match the Tempo, play the notes required, do not worry about how it sounds to you just match tempo and play required notes, and do not "put down" flute.
When performing the song with Rael you may feel that you are totally unable to do it even when it appears that you are doing it perfectly. I found that there is in fct a pause between when he finishes and you start. To avoid this just simply listen the the beat of 4 in the backgorund that plays before you start performing.