See "Against the Odds" to learn how to boost this achievement with a friend.
A weaker team is a team that has less stars than their opponents. For example, Tottenham Hotspur (4.5 stars) is a weaker team than Real Madrid (5 stars). For those unfamiliar with 'Soccer' language, extra-time is the period of time after Full-Time that is played if the score was tied after Full-Time. Extra-Time isn't played in all types of matches but can be in Head to Head Ranked Matches which is what's important for this achievement. You will need to ensure that after the score is tied at Full-Time, you and your opponent select the same option to either play Extra-Time only or Extra-Time & Penalties. If you and your opponent don't select the same option then the match will end in a draw and you will have to tie another 90 minute match for a chance to go to Extra-Time. The achievement will unlock after the second half of Extra-Time ends and you arrive at the post-match stats screen.
Please note that if someone quits out of the game, regardless of if you were in Extra-Time or not, then the achievement may not unlock.
This achievement requires Xbox LIVE Gold and an "Online Pass" or the use of your 7 day Online Pass free-trial.