This achievement has to be done in the classic conflict game mode in a public match. You will need to be rank 35 in multiplayer OR from the main menu go to extras and select cheat codes. Enter the code pa1ntth3townr3d and the mode will unlock. Even as a level 1 you will have acess to 5 default loadouts, one of which has jaws as a playable character. (He is the one smiling a bid toothy grin with metal teeth if your not sure)
Its not enough to be shot in the head, it has to be what would normally be the killing blow. You will survive this shot and see a bonus exp message come up titled braced for impact. Achievement should unlock a few seconds later.
If your having trouble try moving around crouched because jaws is much taller than the other characters so it will put him in better position for headshots.