Like most defensive Achievements, you can easily earn this one by turning all penalties OFF so that you can set up your bronze defender on the wrong side of the line of scrimmage and sack the QB as soon as the ball is snapped. I used William Perry in my attempt. Gotta love the Fridge.
It appears that half-sacks can confuse the Achievement though. My fifth sack was shared, putting me at 4.5 but the Achievement didn't unlock until I reached 6.5 sacks. Just something to watch for...