This achievement is story-related and cannot be missed
*** Spoiler - click to reveal ***When you fight your way through the mansion you will eventually come to the library. Deactivate the Joker's trap and leave the library. You will be effected by Scarecrow's gas again and start to hallucinate.
I Found this encounter with him considerably more difficult than the first one. because you have to time your movements more precisely to survive this.
At the beginning you need to use your batclaw to create new covers for the long catwalk. You will enocunter skeletons that you need to fight twice. These fights are just regular fights and don't require any special attention. At the end you, again, have to use a light beam to beat him. The achievement will unlock right afterwards.
For a detailed description of the timing for to survive each passage, refer to the video below.
(video credit: The Medieval Gamer)
Hope this helps you guys!
Any improvements? Please let me know in the comments below!