Zelda Twilight Princess Walkthrough - Part 10c - Snowpeak Ruins
Break the ice for another clawshot target. Hit the chandeliers one by one and get across all of them for the small key in the chest. Go through the nearby door to the south and then south again to the room with all the freezing guys. Open the locked door to the east and go in.
Go back to the courtyard cannonball department and grab a cannonball. Take it all the way back up the stairs by the cages and find that cannon you've been aching to use. Make the cannon face northeast and load it and shoot it at the ice monster. Goto the ground floor and climb up the ladder where the ice monster used to be. Go through the east door.
It looks like you're in a church or something, but a ton of monsters show up. Defeat them for the Bedroom Key! Heart shaped! YAY! Take it outside and the wife will be waiting for you. Follow her up to her bedroom and unlock it. The wife will show you the mirror shard, and then give it to you. Well, she would, except she really wants to keep it for herself.
Boss: Twilit Ice Mass - Blizzeta
This boss has many phases. Firstly, she is just a mass of ice, so break her down to size with your ball and chain. Second, she spawns a ton of freezing enemies so you need to kill them all and keep hitting her with your ball and chain. Eventually she will have had enough hits.
Her third form is floating above ground and trying to land icicles on you. When they land, destroy a couple because they reform as a circle for her. When they reform as a circle, hit her with the ball and chain. She makes more icicles so keep this up until you hit her three times.
She will be completely destroyed! Well, she's still alive, but she doesn't want the mirror anymore. So you get a Twilight Mirror Shard! Alright! Don't forget to grab the heart piece on your way out, after seeing a gross cut-scene!