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10 G

*Spoiler Alert*

Fairly early on in the story after getting the Savior achievement, you will track in on Joker's radio signal. The closer you get to the Joker, the distance on the signal will decrease (or you can just go to your map and look for the steel mill in the bottom right hand corner of the map...this is your destination). The quickest way involves a lot of grappling and gliding. It is best to sail over the gates of Sionis Industries on your way in since the guards are all armed. Once the distance on your radio signal is down to zero or close to it, Alfred will talk to you about having to find a way in through the main chimney. Look up at the big tall chimney and use your grappling hook to get into it. Once inside, you will have to navigate around the inside until you get to a ventilation system leading to a room full of Joker's henchmen. Once you can see them through the grate, the achievement should pop.

You can't miss this one as it is story related.