#1 advice for all achievements in this game: Set the difficulty to easy from the Options menu and choose easy difficulty in Competition.
Impact Tackles happen at random when you tackle opposing ball carriers and the camera shows the impact in slow motion. The odd thing is these only seem to happen in the very first match you play after launching the game every time and even then rarely. The occurrence of Impact Tackles in subsequent matches in the same session is close to null. Therefore, to grind this out, employ the following tactic:
- set the first match you play after launching the game to 80 minutes (or as much as you can bear);
- turn off High Tackles, Offside and Forward Pass;
- be on the defense as much as possible, when you have the ball, just kick it away to the opposition as you see fit (Punt, Grubber Kick, etc.);
- only perform head-on Aggressive Tackles or High Tackles (Y button on controller);
- once the match is over, restart the game and repeat.
This can take a long while.