To complete Knockout, you basically have to beat 30 teams, who get progessively harder as you reach the end. This achievement is more of a time consumer, as you can't actually lose. If you lose to a team, think you are about to lose, or just want to restart the match, just exit the game and you can start on the team you left off on. It can get a bit frustrating, however, because the last few teams can be pretty difficult.
The strategy I found that works best, is to max out your multiplyer (2x) and then go for the stars. For each star you hit, you will get 4 points (assuming your multiplyer is 2x) and once all five are lit you will get another 20, making 40 in all. Do this a couple times and then just stall the rest of the match, by keeping the ball as far as from your goal as possible.
I would also suggest you pick up as much cash as possible (silver coins), especially in the earlier rounds. That way you can upgrade your players more, to give you a little extra boost when trying to beat those final teams. It is also a good idea to save some cash, so that you can transfer some of the better players when they become available. The farther along you are, the better the players available for transfer.
In my opinion, this game relies more on luck than skill, so keep at it and you should be able to get it, it's just a matter of time.