Batman: Arkham Knight

This is a story related achievement (Potential Spoilers)

During the main story you will be asked to head towards the third island (Founder's Island). Once you get there you have to disable a tower, Lucius will give you a hacking device tool and then you jump on top of the first air balloon. Use the hacking device tool to enter the balloon. After fighting your way through several rooms in the FIRST air balloon you will eventually find your primary target "Stagg". He is being guarded by quite a lot of men (none are particularly hard, just standard militia), once you take them all out talk to him and then Batman will start to hallucinate a bit and all the guards you knock out will begin to stand up. Just run around the room punching them/fighting like you normally would and eventually they will all turn into the joker, continue punching and eventually you will get tackled to the ground and have to 'tap A' to struggle. You will then step back into reality only to see Stagg get taken away (this is when the achievement will pop).

Like I said it's story related so you can't miss it and I'm pretty sure it's like 30-40% through the main story, at least according to my game.

Edit: Ok so I have been told it is possible to miss. My suggestion is to do it as I have stated above, make sure when the henchmen begin to rise that you run around the room attacking them and that when Joker pins you down you 'Tap A' to struggle.