This achievement is glitched. I completed 2 milestones before trying this method to get the achievement.
I found the way to do this is the same way as doing the co-op achievement, completing the necessary tasks in the same session, no lagging out or disconnecting from EA servers (co-op is without either player leaving the co-op lobby). The way I did this was thanks to the new MLS milestone, where you get the 3 MLS special cards. First of all, do this so that you are 1 win off completing the milestone. Then, complete another milestone. I completed the assist mastery milestone as I only needed 130 assists with an 85-rated squad (you can kind of cheat this on squad battles playing against beginner AI), I don't think it matters which milestone you complete. Then, before opening the completed milestone, complete the MLS milestone. I unlocked the achievement as soon as I finished the match, before I even opened the milestones.
Hope this helped.