This is another unmissable achievement. This is gotten on the main mission.
*** Spoiler - click to reveal ***You will get a lead on Scarecrow that he is working at a penthouse in Chinatown. This leads you to Poison Ivy. You fight the militia and get Ivy. After a tank battle with the batmobile, you take her to GCPD.
Note: This is 0-1 hours into the game