BIG WARNING - This achievement is slightly glitched in that your team has to be at the right level *at the RELEASE DATE* of the game. Some teams have, crucially, changed in levels since then, and if you win with one of those teams, it will NOT count!!!
The one we got caught out on specifically was Azerbaijan, who have changed from 1* to 1.5*, but there may be more. We played all the way through, and... nothing...
There's a full list here, double check this before starting:http://www.xbox360achievements.org/game/2010-fifa-world-cup/...
In terms of method, we use variants of Otilia e Gabita's excellent solution which is highly recommended
This is actually possible with a group of 5 or more gamers by tweaking the group stages.
If you have a group with 2 gamers, make sure you win 1 game each (by quit out is fine), and get a SCORE DRAW in your last game (e.g. 3-3 or whatever). You have to play the 3rd game to the end which takes longer, but you will both go through to the next round in 2nd place with 4 points. Why do you need a score draw? The ranking is based on points, then on goal difference, then on goals scored. The other game in your group is random and it's possible although rare for more than 1 team to end on 4 points. By having more goals scored, you will go through. This then means that the same pair can play in the R16 together as well, which means you get 1 quarter finalist for every group play-through.
For every game except the draw, you can quit as soon as the GAME CLOCK is on 5 minutes or more and it will register as a 3-0 win to whoever didn't quit. Matching up is done by setting both of your control settings filters to the same thing - normally 2-Button, but sometimes it will refuse to match you in which case switching to Semi normally does the trick. If you match with the wrong person (rare but happens), back out.
I think the optimal number of players to get this achievement is actually 9, which would look a bit like this: (Winners of each fixture set always on the right)
Player 1 v Player 2 (Player 2 goes to QF)
Player 3 v Player 4
Player 5 v Player 6
Player 7 v Player 8
Player 9 - sit out
Player 2 v Player 4
Player 6 v Player 8
Player 4 v Player 8
Player 8 waits in the final and sits out the next phase
Player 2 v Player 1
Player 4 v Player 3
Player 6 v Player 5
Player 9 v Player 7
Player 3 v Player 1
Player 7 v Player 5
Player 5 v Player 1
Player 8 v Player 1
Player 1 gets the achievement!
And back to the top for the next person in the group. This method is relatively fast per run through - all the games in a round can be played in parallel. There are 2 short group games (2-3 mins each by the time you factor in searching and load screens), 1 long group game (12 minutes + stoppages + loading screens), followed by 1 quick R16 (2-3 mins + loading), then quick QF, SF. That all repeats, then you have the quick final. Should be possible to get this well under 2 hours per achievement if you are organised - not yet had a perfect run through though.
I recommend you sort in advance the fixture order, and make sure everyone knows what's going on - 8 player parties make for confusion, and of course there's 9 of you anyway! We actually found it easiest to have 1 organiser in his own party, and then each match pair form their own party, and then join the organiser party when they are finished if not sure what's happening next.