When you're playing online at the Map: Villa go to the house with the Bar counter on the first Floor and Fedelle Castro on the wall, there is the pool outside.

Go up the stairs and go to the window. Shoot the glass and climb onto the window pane.

Look to your left, you will see something on the wall (I don't really know what it is, but it looks like a huge flat box against the wall).

Go as far back on the window pane as you can, sprint forward and jump into this Box thing. You will be stuck in this and the other players will not be able to see you.

Equip your sniper and just kill! You will be able to see everything but the other players will not see you.

Note that you should choose a sniper for this, your secondary should be a China lake Rocket Launcher.