Spoiler Alert:
This is from completing the side story involving the weapons caches (as is fairly obvious from the description). After you complete the 4th weapons cache you will immediately get a clip showing Nightwing being held hostage. You will contact Alfred and he will use the tracker to find Nightwings location. Follow the objective marker to the TOP of the roof and take down all the armed guards up there. You will then break the panel for the elevator and continue to drop down the shafts until you reach a room with a sentry gun. Use your hacking device to disable briefly and then destroy it. The door next to the sentry gun is locked so proceed into the vent directly opposite. Follow the vents through the room where Nightwing is into the wall behind him then press yourself up against the wall and you will get the "Dual takedown" option. This will start a fight scene, knockout all the guards and then proceed to spray explosive gel on the weapons cache. Once you do that and have closed the door you will turn around to see that Nightwing has been apprehended by Penguin. After some dialogue you will get another "Dual Takedown" option, then some more dialogue and finally you will be tasked with taking Penguin back to the GCPD. This achievement will unlock shortly after he is placed in his cell and you once again get control of Batman