Use this guide or my written guide.
Credit: Batman Arkham Videos
After saving and returning back to where Joker is, you need to use your Remote Electrical Charge to power up the hook, and swinging it properly, you are able to smash the door. Pull the hook and finally enter the manager's office. A boss fight comes and you have to defeat a lieutenant with a hammer, try not to get hit. Remember to use evade and I recommend taking out the goons first. Once defeating him, you have got the steam achievement.