Very easy to do in training.
Start the multiplayer training program.
You'll have to kill 3 targets to complete it. Kill the first two quickly because only the third is relevant for this Trophy.
The third target is blending in a small group of people at a market stall. Keep a low profile, blend in behind him, lock the target with
and wait for the focus meter to be completely filled before killing him.
If you did it correct you'll receive exactly 750 points and the Trophy just seconds later.