Start Police Academy on Hard Mode. Make sure you have Mobile Cover equipped with the Extension mod.
Run down the street until you get to the double doors where the enemies blast the doors open.
Drop the mobile cover at an 90 degree angle and take cover on it and walk through the doors.
Proceed backwards to the echo on the floor and click it, which will trigger the mission to that point.
Basically go about the mission as normal and kill the boss.
After you kill the boss head back out of the mission. Closest way out of the mission, if facing the double doors is go right and near the cars reset mission and rinse and repeat.
Created goes to BreezyPeazy