This is earned for completing the most wanted mission entitled "The Perfect Crime."
Throughout the game, you will receive reports of opera music being heard, which will pop up as the side mission. In the area where the music is being played, there will be a body strung up that Batman will need to identify. The louder the music gets, the closer the body will be.
The process of identification is a three-step process. Activate the evidence scan (
), and you will have to go cycle through the three-different layers on the scanner (
and
). There will be a piece of evidence to find at each layer, somewhere on the body, and it is highlighted and stands out from the rest of the corpse. Once you find it, hold down
to scan the piece. Once you have all three, you will have cleared that victim.
There are two victims on each of the three islands for a total of six victims. On Bleake Island, the first victim is already highlighted for you in the most wanted mission select. The second Bleake Island victim is located in the southwest corner of the map — if you zoom in on the map, it's just to the upper left of BLEAKE ISLAND. Looking at the map of Founder's Island, the first victim is to the right of the top of the airship. The second Founder's Island victim is on the east of the island, west of the long, skinny pier. The first Miagami Island victim is near the Miagami end of the Salvation Bridge, just northeast of the bridge's end on the map. The final victim is in the northern area of Miagami Island, on a building with smoke coming out of the roof.
Once you identify the final victim, Alfred will direct Batman to the killer's lair on Founder's Island. Without spoiling the identity of the killer, you will need to defeat about a dozen of his minions before being able to stop him and rescue the victims.
The problem with the minions is that they are impervious to pain — if you knock them down, they keep getting up. You will need to do a ground takedown on them (
+
) — even the combo takedown of
+
won't work — to put them down permanently. While you fight the minions, the serial killer will be throwing scalpels and knives at you — you can counter them as they come at you, and Batman will throw them back and stun the serial killer. Once all his minions are down, you can perform a takedown on the killer.
After freeing the victims in captivity, you can pick up the killer and take him to the GCPD lockup, popping the achievement.
EDIT: Some people reported trouble with the takedown not being the same as the on-screen prompt. I've included this info in case it's a glitch for others.
"You have to press X and A simultaneously while the object he's hiding behind is highlighted in blue. (For some reason the game is giving a useless and irritating clue to press LT and X, which does nothing useful)."