Grab the ammo and fight it out.

You see the elevator but you can't get to it.
To get the Gold Key move straight forward from where you start.
Then you venture into the southern parts.
The Silver Key is hidden near the central room and so is a 1-up.
Try to shoot as many as possible in the central room past the column or the well by the treasure.
Then take out the rest on your way to the elevator. You have to push a wall block in the central room.