Mission 6: Legends Never Die
During the mission, the city will flood and then you will be prompted to kill an enemy using a knife. Before you kill them, make sure that you are underwater. Moving on in the mission, you'll come to an area with some wreckage, where you should be able to sneak to the back and kill at least two enemies using the knife underwater. Lastly, you come to a parking garage, where you should sprint to the back as fast as possible, and knife at least 3 enemies underwater. If there are only 2, just wait, and another enemy should jump into the water.
The achievement unlocks once you knife the 6th person.