Read carefully:
At the beginning of mission 12 when you get the SSR 4000 use that to aim, then use another weapon to kill the men on the towers. You need to save the SSR 4000 ammo! Get their ammo and once you do that, do as you normally would to get to the World Leaders. Get through the silos then shoot Bloch A LOT with your D17. After you shoot him, shoot him twice in the face with your SSR 4000. He will drop his MRL-22, then he will fly up to the pipe on the top floor and crawl into the pipe. The you and he will appear at the beginning of the Cold Reception Level. The floor will start to crumble. Watch out for the holes! If you fall in... it's over. You must shoot him with your MRL-22, and he'll die.

If you need any more tips, I can try to help you out.
My email is:
Soccerkix1139@aol.com

Later
-Matthew