To get this achievement you need to have played through all the missions, bought all the characters that can be bought and you should be left with 2 to get.
All you need to do now is rescue all the hostages and that allows you to purchase Hush.
P.S. The other character that is left is Ra's Al Ghul who is available to buy only after you have 100%ed the game. I have no idea why you don't need to purchase him to get the achievement.