This should come fairly easy when playing on at least Officer difficulty. Cadet offers no multiplier, and some of the levels you will have to be almost perfect on in order to score silver.
If you die, hit the dashboard button and reload the game from the last checkpoint. This will help your timed score. One of the bonus categories I frequently missed was the "counter" one. Just find some lone enemy at some point in the level and counter him 5 times before you kill him, for a quick 500 points. Use Shadow strategically but not liberally (you don't want to lose your survival bonus) so you can score the "Tag Team" bonus. Besides that, headshots and takedowns, and you'll be fine.