I'm level 27, I have 3 out of 4 of the Hall of Echoes items.
I lack the Key.
Then you'll need to go to the mine in Broken Valley... but you might want to level-up at the flying fortresses for a while. I went to Kali's Fortress (the one with the L29 enemies) at level 27 and came out with level 32.
By the way, in the Lost Tomb, all there is to do is to pull the levers and get what's inside the chest, right?
Is this the one you enter as part of the tomb raiders quest? Then yes, there's nothing to do except pulling the levers and opening the chest. Most of the time, there's nothing special in it.