Pet Rescue Saga Level 565
Target: Rescue 8 pets
To lower the key in this level you first need to get a bomb in the bottom row, diagonal to the metal boxes. Once the lowest metal box is gone, the fire and ice bombs over the other metal boxes meet and defrost, so you can remove those boxes too.
It can be easier not to unlock the key immediately though.
Wait until you have your pets down as low as possible, and (if you can) move some bombs in under them.
Then, unlock the key, blow up the bombs and rescue those pets!