This solution uses Game Settings to make unlocking this achievement easily. Please do not down-vote if you're a FIFA purist and insists that "everything must be done legit".
This is an extremely easy achievement if you abuse the Game Settings prior to starting the game. Pick any team you want to control, and make sure that whoever you're playing against must be of the same team rating or higher than you. For example, I use Liverpool (4.5 star) vs Fulham (4.5 star). Do the usual and go into the match menu.
Once inside, select "Game Settings". Scroll LT/RT until you reach the part that says "CPU Settings". Here, you'd find all the settings (in the form of sliders) that the CPU would be using against you. "Slide" all the useful settings (e.g., passing, shooting, sprint, acceleration, etc.) to ZERO, and set all the not so useful ones (e.g., GK error, pass error, etc.) to 100.
NOTE: If you want, scroll LT/RT to "User Settings" and slide everything to your favor. That's not really necessary though.
Obviously, remember to set your difficulty to Legendary too, since this is what the achievement requires.
You'd now be playing on Legendary difficulty against a team of duds. The opponents would be so weak, you won't believe you're on the hardest difficulty level of the game. I ended up winning something like 10-0 to unlock the achievement.
You can pair this up with this achievement:
The Comeback Kid achievement in FIFA 12 worth 31 pointsWin after being down 3 goals in the 2nd half in a game vs. the CPU
by deliberately scoring three own goals to gift the opponents a 3-0 lead. Start scoring only in the second half. Even with a 4-minute half timer, you should easily bang in tons of goals to unlock both achievements.