After you complete Co-Op on Mozdok grounds and are in the plane there will be a final Mission where you have to Kill your Co-Op teammate.
This is quite easy to get for both players. When the Kill Mission starts both players run to a central point, Player 1 kills player 2, as soon as the achievement pops, (Start) (Restart Last Checkpoint) repeat. Player 2 kills player 1, achievement pops, mission ends and still will get:
If you have been playing on Realistic throughout the entire Co-Op.