From the Main Menu select the UEFA Champions League mode. Select Real Madrid as your team to play with.
Once the tournament has loaded you can just simulate each individual game instead of playing it. Press
when you have the play match option highlighted to do this. Now just sim every game and Real Madrid should walk it. You will be very unlucky not to qualify.
Save after each win if you need to. Will unlock after the 6th group game has been done.
Edit: Seems you have to win the group stage to get the achievement now. Doesn't change the solution, Real Madrid should still win the group easy. If you don't win a couple of games then back out and load from the last save.
This has now been patched and you do not need to win the group to get the achievement, just finish in the top 2.
Thanks Darth James and Miarches.