This is really not a hint it is just well common sense. In the early levels make sure you get rid of every enemy to make sure you level up to the highest. Make sure you get rid of the red ones before you start on any targets. Also do any free missions when you unlock them. This can also help to find you new weapons. There is only one exception and that is timed missions, but still try and get rid of as many as possible it will make the later levels easier.