Co-op is where you and another person both control the same team against either a CPU or human opponent(s).
From the main screen within Ultimate team, press
to bring up your friends list and press
on the friend you wish to play co-op with. Then, when they have accepted the invite, you will be able to select Squad Battles, Division Rivals or Friendlies. All 3 game modes count towards this achievement. Once you have completed 10 games with a friend, you will pop the achievement.
Edit: this achievement appears to be bugged and is not popping after completing 10 co-op matches for some players.
Edit 2: to anyone who down votes or disliked this solution, please let me know why! I don't often publish solutions so some feedback if it isn't good enough would be useful 🙂
Edit 3: as per JJR0906,
I've seen someone say in a YouTube video that you need to do ALL ten games in the same lobby/session, without one person leaving. Not sure if this is true or not
Might be worth trying this if you haven't already. I did mine over 2 sessions though so it shouldn't be a requirement.
Edit 4: as per EwanJStewart,
Can confirm that this achievement unlocked for me the second time round after playing 10games in a single run.
If you've played the 10 games and it hasn't popped, definitely try doing 10 games in a single session as for some players this has worked.