On this page of our game guide to Batman: Arkham Knight there is a description of Lamb to the Slaughter side quest. It consists of difficult battles and solving the puzzle with the generator. This mission also forces you to race against time - you may fail the mission if you're out of it.
You gain access to this mission as the last one and it takes placer towards the end of the main storyline. After you have completed the Stop Scarecrow's Men from destroying Oracle's Servers main mission, Batman will be contacted by the Oracle. From her, you learn about the trouble Jack Ryder has gotten himself into.
To start this mission, you need to reach an islet with the statue of the Lady of Gotham, which neighbors Bleake Island and Miagani Island. You need to glide there, because the islet with the statue is surrounded by water on all sides and there is no access route there.
Approach the blip on the radar and enter the detective mode. Thanks to this, you will learn that Jack Ryder is being held hostage by Deacon Blackfire. Land on the arena, where there is a big group of cultists and start the fight.
In total, you have 3 minutes to perform all of the activities so, you need to hurry. Eliminate the goons, one after another, and first take on the ones trying to use their firearms. Stay away from the electrified fences surrounding the arena and seize each opportunity to push the opponents towards them.
After you have handled all 25 goons, select the electromagnetic pulse from the inventory. Use it to disable the four generators that power the fence. After you do that, use the grapple to catch n to one of the grates marked on the screenshot. Thanks to this, you will reach the area where deacon Blackfire is staying. Finally, see the cutscene of imprisoning the deacon.