Yes the blocked punt method works great - I got both FUMBLE Achievements from this. Eventhough I was not credited for two fumbles in the stats during the game - I got the SEASON and BUTTER FINGERS ACHIEVEMENT anyhow. Just turn off all penalties so you can line up behind the center before the ball snaps. Of course making sure you use the same guy every time. I like the guy that is lined up the most left ON THE LINE. Then pull down just a little bit - nice and soft on your L-stick so it looks like your player is in slow motion running towards the punter. This way you have immediate control over the player - sometimes your guy will go crazy if you just line him up close.
There you go!