Madden NFL 07

I should have this finished soon currently 15/20
Done! talk about limping over the finish line. This one sucks and your going to need some luck on your side that's all I'm going to say...

Like others have said I think a key aspect is to make sure your dad is a "hall of fame" player in the position you are going for. I tried with a "pro-bowl" dad at ROLB and was ok for the first few years, getting 1-2 FF but then it dropped way off the third year or so and was stuck at 5 forced fumbles. so I tried another player and made sure I had a Hall of fame ROLB dad. not sure if it matters but I also scored 90% on my IQ test. doing nothing but simming entire seasons here's how my player breaks down.

Position: ROLB (I found the line backers i.e. LOLB, MLB and ROLB end up with the most tackles on the team at the end of the season and it just makes sense that if you are getting the most tackles then you have the most opportunities to force fumbles)

Father:Hall of Fame ROLB

Team: (don't know if the team matters) Drafted by Saints


Stats
Years pro/FF for year(end of season stats)/ total career FF

0(rookie) / 3 / 3
1 / 3 / 6
2 / 3 / 9
3 / 2 / 11
4 / 2 / 13
5 / 2 / 15
6 / 2 / 17 ** (first time simming this year i didn't get any FF so I reloaded and simmed the year over to not waste a year)
7 / 3 / 20!

Achievement pops when you advance to the off season I believe, I was so done with this game I forgot to write it down, credit to the first person who can confirm when it pops I'll update this solution.
Confirmed - https://www.trueachievements.com/X+PureMcNasty.htm - "I can confirm that the achievements pop up in the offseason."

Good Luck!!! and may the odds be ever in your favor!

I was originally just going to post on a comment on NickHawkEye's Solution but this actually worked so well for me I thought it deserved its own Solution.

After working on the 140 tackles in a season, I ended up with 8 Forced Fumbles that year. So I just kept that career going hoping to sim my way to the 20 Forced Fumbles.
I wasnt having as much luck as NickHawkEye with the season sims. I was only getting 1 a year. So after several seasons I begin to worry my guy would retire on me so I started trying something different.

I simmed just the first month. Got 0 FFs the first couple times, so I just backed out and reloaded, then 3 FFs in the first month. Saved, then repeated for the next month. Same thing: 0 FFs the first couple times, then 3 FFs again. Save.

I repeated this for each month, then let the season sim out once I got the 8 FFs I needed for the achievement. But patience with this method could easily net 10+ FFs a season.

Once you get your 20 FFs the achievement will pop once you sim out of February.

I was having a difficult time getting the forced fumbles bu using the above technique. I was getting around 20 sacks a game with averaging less than 1 ff a game. I finished the season with 10ff. When I started the next season I opened up with 3ff in my first game. So my advise is not to get frustrated and if anything maybe try to sim to the second or third season and start attacked the qb and punter as stated above.