Start an NRL or SL season with high tackles set to off. Commence the first match. Score as soon as possible, then focus purely on getting impact tackles. When your opponent has the ball, use in every tackle. If they kick or drop the ball, give it back to them (by kicking). Any player is capable of an impact tackle. They happen at random, though there are a couple of things you can do to increase the possibility of pulling off one of these tackles.
*Make front-on tackles with the user controlled player. Whilst the computer can occasionally produce an impact tackle, it is far more probable when you are the one making the tackle.
*Go to the dashboard after each game. I don't know why, but the first match after booting it up results in more impact tackles than usual. 3-4 on average in a 10 minute match.
(Credit to Tweekfu)