Another sacks Achievement, another excuse to turn off the offside penalties...
Start a season mode game, turn penalties off, and when you are on defense, position your defender across the line of scrimmage and next to the opposing quarterback. Be careful not to touch any other players, or you may still be called for encroachment. Use the same player each time so that all the sacks will be attributed to the running total.
You need 23 sacks in the season. If you crank up the length of the quarters, you may be able to get this in a single game using this method. Just punt the ball away on your first down to keep yourself on defense as long as you can until you unlock the Achievement. If need be, you can continue working toward this over multiple games. Along the way you should be able to unlock the Use a silver-star defender and you can also earn the "Game Record for Defensive Sacks" Achievement (at 8 sacks), and the "Bring 'em Down" Achievement (after 15 tackles - even if they don't count as sacks).