Blitz The League

Another way to pull this off is during the Division III season, I made my Safety the veteran (others used the LB (linebacker) as the veteran) and had my team gain a 14+ point lead and then I sent my Safety after the QB on several blitz formation, especially using the Crush > Shark formation and flanked the QB on either end of the line and sacked the crap out of the QB whenever possible, I was averaging 4 sacks a game and the achievement unlocked after winning the Division III championship game.

Please note: After some testing after I popped the achievement, using the Linebacker would be a better option, got 10 sacks in 2 games using Ted Lawless and having the Defense Coordinator that makes LB stronger and faster.

The achievement for this will pop after the game where your Veteran achieves 20 sacks in a season. If you want to know how many sacks your vet has check the statistics section inbetween games.

The best way to get 20 sacks in a season is to use the guide I provided under the Manvalance achievement. I know this isn't much but you will find what you are looking for there.