Clear the main hallways, then get the Gold Key.

There are SS in some of the rooms, but they are behind stationary objects.
To get hold of a machine gun as well as some treasure you must get past pushwalls.
The best way is to kill the SS at spots where they won't block the pushwalls,
it is still a little difficult though.

Grab as much treasure as you can plus the Silver Key and head on towards the elevator.