Instructions
Walk toward the far wall of your cell after dropping into the prison to initiate a conversation with the man in the next room. Once the discussion has begun, press "X" repeatedly until the discussion and cut scene are over and Snake is in the dead prisoner's cell.
Crouch and crawl under the bed to find a ration, and then wait for sounds of a struggle outside your cell, at which point the door will open. Press "X" to bypass the next cut scene and conversation, or wait for it to play out.
Equip your Socom and shoot the first three guards. If you do not have a Socom by this point, you'll find one on the floor next to Snake.
Stand in the far-right corner of the room and fire on each successive wave of guards as they enter the room. In between waves, venture into the middle of the room to pick up ammunition and rations. Replenish your health by selecting a ration and pressing "X" if Snake is close to death.
Avoid the grenades that the guards throw into the room by hiding in the upper-right corner. Shoot the last round of guards then walk to the elevator. Select floor B2 to leave the prison and proceed to the game's next level.