Beat Scatter Gun course was a mission in Jak II. After making a delivery Jak is rewarded with the Scatter Gun by Krew, who said that if he could prove himself at the gun course he might hire him for a job or two.
First follow Sig's instructions, where after the actual course opens. You'll need to get 3000 points where after you can practically run to the end of the course and complete this mission. Just shoot the cardboard Metal Heads (especially the gold ones) while avoiding any civilians.