By now you will have a machine gun, and you'll need it.
The enemy will not give you much time to think in this level.
You can move around a lot in this level and so can the enemies. Prepare for surprises.

Get the goods in the north-western area and try to shoot as many SS as possible past the columns. Move through the secret door to get the Gold Key.

Similarly the Silver Key is in a secret area in the south-western part.

Then get ready for the elevator. There is a 1-up not far from it.