If you are sniping.... remember the AI is stupid.
If the AI you are trying to hit is far away and moving, dont worry too much about hitting him coz if you get close he will stop.
Then just headshot away, and watch those kills rack up.
Stormtroopers (and others similar) will roll after the first shot... and then stand still. Generally different troops act slightly differently, but you learn quick enough and can pick them off.
If you are going on as a grunt, the Z key is your friend.
Zoom in, strafe and aim for the head. About 6 shots at most should finish them off.
If the crosshair is blue, then you are probably out of range, as soon as it goes red blast away. Usually they kinda of stop, or roll and stop, after thats its candy from a baby.