Get inside the checkpoint and type /mission to start the mission. Once the mission starts you will get a dialog of the top players who scored in this mission.
Once the mission starts, you need to start it by shooting the target that is visible. Once you shoot the first target, the counter will start and the mission will begin. You will have to shoot all the targets that are visible in 60 seconds. The more targets you take down, the higher you score. You will receive cash per target.
Once the mission ends, you will unlock the super aim achievement. You will also be given score and cash based on how many targets you shot.