First go to terrorist hunt on killhouse on medium difficulty and high density.

Then when the game loads up walk through the door where you start and send your soldiers straight, but you turn right and lean against the right side of the doorway.

Then roll a frag into the hall and as soon as it goes of the terrorists will start to say random and strange things like: "check it out" and start to argue with each other.
Then head out of the door and lean against the wall, shoot the guy and go upstairs.

Tip: the p90 with laser is the best gun for this cheat, also have frags and incendiaries for funny moments!