Difficulty: Easy
Play a large scale game on Caspian Border, like Conquest, and head to
point D.
To simplify this you'll want to do a server search so you can find only games on this map.
There's a lake around one side and a barely visible pipe in the water.
Dive at the pipe and swim through. You'll get the trophy at the other end after a bit of running (It's not too far at all).
If you want to see the pipe then here's a decent video walkthrough: