When you are about to go to the third floor instead of going there go straight. You will fight a dragon. There in a cage is the boss key.
Find the big key.
in the main room
Solving puzzles and deafeating enemies.
once the yeti takes you to all the wrong rooms. she'll finally get it right. next to the bedroom (aka the boss room) the key is there on a hill.
You don't get the mirror shard from a chest. You have to defeat a boss to get a mirror shard.
you have to find the three key shards which open to the boss gate its very easy
Follow the trail on the ground
In every dungeon, there is a large, pointed, sinister looking chest. Inside is the Boss Key. Usually you have to solve a puzzle or defeat a monster to get to it. If you find the Dungeon Map, that will help you quite a lot.
If you want to find the BOSS for the Lakebed Temple on Twilight Princess, you have to get the BIG KEY and then go to the room with the giant swiveling staircase, and just jump into the water. The BOSS DOOR is level with the water surface under the swiveling staircase. This BOSS is the coolest one so far. -Caveman1593
go get the big key....
there are three or four (if not more) big keys in the Twilight Princess game.
Look on your map. There is a room directly above the chest. You could hookshot a medallion and lower yourself to the chest.