You got into the ventilation (#215). Follow Griggs and when he turns, join Price. Go will get to the baths (#216). You will learn that 11 minutes is what you got to neutralize the launched missiles.
When you get out of there, turn left and go straight (#217), eliminating the enemies. You just have to keep going and do not waste too much time on shooting. Use as many grenades as you want - the faster you get rid of the enemies the bigger chances to neutralize the missiles. You should get to the stairs (#218).
When you're in the next part of the base, do not go straight. You should turn left or right(#219). You have better chances there. Use the flash grenades, especially in the corridors where you can get shot from the back.
The most difficult situation is in the silos with missiles. Hide behind the missiles and watch out for the fire or steam coming out of the pipes (#220).
When you're at the end of the path you will have only seconds to get behind the armoured door (#221). Run as fast as you can. Shortly after Price will close the door.
Gaz will open the door leading to the next part of the base (#222). As soon as you are able to push through the door, do it and start eliminating the soldiers. Then you will have to plant the explosives in the certain point (#223). Detonate C4 and get to the control room. Kill the rest of the soldiers and head to the keyboard (#224). After you give the right codes, the missiles will be neutralized.
You will have to evacuate then, so follow Price. On your way out you will have to kill some soldiers. At the end there's a lift (#225). When you get up (#226), help with exterminating the enemies head towards the exit.