To the Boss Room

Main Room: You'll find yourself in a room with a large rotating flower in the center. Stand on it, then aim a regular arrow at the unlit torch in the northwest corner. Wait for the flame to sweep across your path, then shoot the torch to light it. This will open the final door ahead.

Final Room: If you've been collecting everything like a good hero, you should have 11 Stray Fairies already. First, take out the Dragonfly and Skulltulas hanging around. Now, equip the Great Fairy Mask. Shoot the bubble floating in the back left of the room to snag Stray Fairy #12. Next, switch to your Deku Mask and head to the eastern alcove to grab Stray Fairy #13. Keep climbing up that eastern side; you'll find Stray Fairy #14 waiting for you. Finally, make your way to the west side of the room to collect the last one, Stray Fairy #15. That's all of them!

To the Boss Door: Now, look across to the east side of the room. Fire an arrow at the crystal there to douse the flames blocking your path. Use the Deku Flower to fly into the covered area, then use another Deku Flower to reach the pillar, and finally, use a third Deku Flower to reach the eastern roof. From this roof, use the Deku Flower to fly over to the ledge where the boss door is. Unlock it, and head on through!