Step One. Throw your marker at your feet and shoot the dummy three times with your m4. 


Step two. Stand behind the dummy, throw your marker at the dummy's feet and respawn as fast as you can, and run into the course.


If you did it right, the course should start with no dummies up and the timer running. Now run until you get to the door at the end.

Step 3. Now go here and throw your marker twice until the dummy dies. Wait until the countdown ends, then enter the door, and the achievement's yours.