This is gotten during the single player campaign and will constantly build up through all restarts, deaths, and multiple levels of difficulty. So no need to worry about your score dropping!!!
You need in total 16000 points to get to level 5, and points can be earned in a variety of ways. This game does favour you taking a stealthy approach, with generous points awards for doing so. I have written a short list below of my main methods of point accumulations, and jotting down what I remember is roughly the points total.
Alarm disable: 200
Arrest Warrant - Non Lethal: 1000
Takedown (Taser/no freeze): 100
Enemy Spot : 20
Explosives/Alarms spot: 20?
There may be others available, however these seemed to pop up rather often for myself. I would say the majority of players should, if they follow a stealthy approach, get this achievement by episode 4.