One Shot One Kill is the fourteenth mission in the Call of Duty 4: Modern Warfare Campaign.
Use the Barret 50.cal and wait until Zakhaev gets there. When MacMillian tells you, shoot him, but do take the wind into consideration. Just watch the flag on the car. If its flickering too much, move the crosshair to the left or to the right (depending on the direction of the wind).
When you shoot at Zakhaev, he will lose his arm but according to MacMillian that's enough. Then shoot once more, this time at the pilot sitting in the helicopter. It will get pretty hot then, and you will have to run. Follow the captain , grab the line and go down. Enemies will show up. Kill some of them and when MacMillian says so then continue running. You will get to a building, in which you will be able to hide for a while.
Wait for MacMillian to open the door and then go straight ahead. After a couple of seconds, a wild dog will attack you. Kill it and keep moving. Soon more enemies will show up. To make it easier for yourself, plant some claymores by the front door. Before you leave the building, pick up the enemy's weapon – it will be useful soon. Kill the next enemies that attack you and then shoot at the helicopter. It will crash and pin down the captain.
From this point you have to carry your commander, and this means you will be completely defenceless. MacMillian will help you with killing enemies but first you will have to lay him on the ground. Soon you will have a chance to try that method out as enemies will show up. The second time will be in a small park. Enemies will attack there again. But this time there will be more enemies and they will attack from many different directions.
To finish this mission you will have to supress all the attacks until a helicopter comes to pick you up. This task will not be easy as terrorists will be coming in great numbers. I suggest staying close to MacMillian and eliminating coming enemies with the sniper rifle. If anyone tries to get to you from the other side of the ferris wheel, use the weapon you picked up a while ago to kill them off.
When the helicopter is finally there, pick up MacMillian and place him inside. The mission is now complete.