Try to always allow a gunman to draw, as you only score 100 points if he doesn't. If two or more gunmen appear take one out straight away, as it is a lot more difficult to kill more than one when they draw.
You don't have to shoot the little guy with the hat an exact number of times, just once for each hat or more. So fire as many times as you can and you will get the bonus points.
You should try to defuse bombs as soon as possible.
Doors cannot be opened while you are moving, so move directly to the door you require without stopping.
Watch the indicators above the door numbers to see which doors will be opening soon. If there are two next to each other, try to get only one of them on screen so you do not have to possibly contend with more than one gunman simultaneously.