At the very beginning of the mission ‘One Shot, One Kill’ you will have an opportunity to take part in one of the BEST achievements I have ever seen in Xbox history.
As you are asked to locate Zakahev (a bald guy in a brown jacket) you will notice a vehicle to your left, this vehicle will have two NPCs in it. One in the front seat (this is Makarov) and one in the rear.
You must wait for the helicopter to block your view and then fly away. Wait for the wind to die down and MacMillan will then say ‘it’s now or never’.
At this time, shoot Zakahev, then quickly shoot Makarov in the front seat of the vehicle.
You can use multiple shots and you can do this on any difficulty.
This, of course, allows the game to match up with the events of MW3. Which is so freaking cool! As Infinity Ward went back and retroactively added in the NPC and this achievement to make the story work!