The online component is broken beyond words and full of quitting scum so forget about doing this legit. Instead you'll need a boosting buddy if you want to get anything done in this game. Your biggest enemy is lag. It makes all the players move like trucks on ice and it makes nailing a pure shot nearly impossible.
The crests come in two flavours. Club and National. Do the National ones by winning three matches with each nation (no 'legend' teams required). Do this in 'Play A Friend' mode. Settings: easy, win by 2. The losing player just needs to score two own goals. Don't complicate it by trying to score proper goals. I suggest each person does their three wins and the you swap the winner over. It'll take a few hours for both players to get all their National crests.
Once you've got that out of the way, you need to boost 'Play Anyone' matches - you can do the remaining crests in 'Play Anyone' but you might as well make progress on getting into Division 1 for tthat achievement. Just go in at the same time and you'll probably meet. This game is a ghost town online.
Once you meet the strategy changes a bit. Now you need to score loads of goals for your club challenge crests. Scoring legit is horrific because of the lag, even if the other player stays still, so concentrate on the losing player running the ball out for corners.
Corners are easy to score from once you memorise the exact amount of time it takes to get a pure shot. Concentrate on this, more goals = more multiplier = more PP. 'Play Anyone' matches are set to five minutes. Once you get your technique down, it's easy to score at least ten goals.
Don't forget to go for the other achievements (win three games in a row, win a game with a clean sheet after losing a game) as well.