Scars of Velious Progression Achievement 'Keys of Frozen Shadow' Unobtainable

The Keys of Frozen Shadow achievement includes all 6 normal keys but also includes the 7th master key. If you have used all 6 normal keys, the master key is never used so is never put on your keyring for the achievement.

Tserrina's Key should give the achievement OR the other 6 keys.

You use the master key by clicking in the mirror room on the first floor.

To add to what Soulbanshee said, after crossing the center room on the first floor with the pit, take a left instead of right. Right goes to the mirror for the second floor, and left goes to the mirror for the mirror room.
That one mirror is the only thing Tserrina's Key is good for. Her key doesn't allow you to click up to floors for which you're not already keyed.

I had been hanging on to T's key for some time. Tried many different way to "add to key ring", thus granting the final achievement. "take a left instead of right." Dang! I would have never figured that out!. I now have that achievement done!! Thanks Gialana!