Certain players in FIFA 11 has traits/ specialties attached. These include "playmaker", "crosser", "aerial threat", "distance shooter" and "poacher". If you perform certain tasks with a player with these abilities, you'll get an achievement.
One of the ways to get the "Crosser" achievement is to play as Manchester United and put Ryan Giggs in the starting line-up. Keep crossing the ball with him and if someone scores a goal from his cross, you'll unlock this achievement.
You can do so on any difficulty level. I won this achievement on Amatuer.
PS: Since Nemanja Vidic (aerial threat) and Dimitar Berbatov (poacher) are in the same Manchester United team, you could do all three achievements in one game.