The issue with using setDammage 1 is that all body parts take on their wounded texture, which isn't the (Hollywood) calling card of a masterful assassin.
You could try RKurtz's enhanced sniper method. It would look great in slo-mo with views of the sniper firing.
My suggestion would be to spawn a bullet and hit the target with it. The following should give a puff of blood on impact whereupon the victim falls dead, but I wouldn't rely 100% on it - check half a second later that the target is dead and, if not, then use setDammage on him:
A very simple scenario: the target is on the ground, facing South and is named patsy. The script is as follows_bullet = "BulletSniper" camCreate [ (getPos patsy select 0), (getPos patsy select 1) - 1 , 1.2 ]
_bullet setVelocity [ 0 , 200 , 0]