it's where the spin thing is
The boss key is in the room where there is water under u. U will see the chest with windmill like things around it and a wooden gate. Look on the ground to see a pathway. Use the gale boomerang and blow the windmills in the exact order of the path to open the gate and get the boss key.
The walkthrough at ign.com has that information ready for you.
It's on the map.
beat the boss
in the main room
in the main room
You finish off the forest temple mini boss
You get it in the sky temple when you beat the mini boss.
Yes, But you can't fight the boss again.
The mirror of twilight is in the desert. you gain access through the mirror of twilight after defeating the boss of the desert temple.
as soon as you defeat the Minny Boss in the Forest Temple you get the boomerang
kill the lakebed temple boss or collect enough heart pieces
You have to beat the mini-boss in the first temple, which is Ook, a white monkey
collect all the monkeys then defeat the flower boss