Okay really easy achievemet can be one in about two minutes if you follwo this guide. I had no intention of playing this mode so worked well for me.
Start up the mode, if you don't know how it is under football life once you have done that go to my team and release a player of your choice buy pressing A on their name and selecting release player, follow the on screen confirm this and one you ahve doe that you should recieve a credit of roughly £500,000. The reason you do this is so you have enough money to sign a player.
Next go to sign player and then choose show all players, hit X to sort and select Catergory - Trasfer Fee and Order - Ascending once the list has loaded simply select the top player hit A on sign player pay the money (you should have enough if you released a player) and thats it achievement unlocked.
Thanks for reading hope this helped